Optical Properties of ZnO Nanostructures _reviews (RIP) ^_^_^_^

Zinc oxide

DOI: 10.1002/smll.200600134

Optical Properties of ZnO NanostructuresAleksandra B. Djurisic* and Yu Hang Leung

From the Contents

1. Introduction.............945

2. Spontaneous Emission................................947

3. Stimulated Emission 951

4. Nonlinear OpticalProperties................955

5. Optical Properties ofDoped ZnO...............956

6. Conclusions andOutlook....................957

Keywords:· nanostructures· photoluminescence· spontaneous emission· stimulated emission· zinc oxide

Many morphological variations of nanostructured ZnO lead to some interesting optical properties.

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reviews A. B. Djurisic and Y. H. Leung

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We present a review of current research on the optical properties of ZnOnanostructures. We provide a brief introduction to different fabricationmethods for various ZnO nanostructures and some general guidelines onhow fabrication parameters (temperature, vapor-phase versus solution-phasedeposition, etc.) affect their properties. A detailed discussion of photo-luminescence, both in the UV region and in the visible spectral range, isprovided. In addition, different gain (excitonic versus electron hole plasma)and feedback (random lasing versus individual nanostructures functioning asFabry–Perot resonators) mechanisms for achieving stimulated emission aredescribed. The factors affecting the achievement of stimulated emission arediscussed, and the results of time-resolved studies of stimulated emission aresummarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnOnanostructures are also discussed, along with a concluding outlook forresearch into the optical properties of ZnO.

1. Introduction

Zinc oxide is a material with great potential for a varietyof practical applications, such as piezoelectric transducers,optical waveguides, surface acoustic wave devices, varistors,phosphors, transparent conductive oxides, chemical and gassensors, spin functional devices, and UV-light emitters.[1,2]

Its wide bandgap (�3.37 eV at room temperature[1]) makesZnO a promising material for photonic applications in theUV or blue spectral range, while the high exciton-bindingenergy (60 meV)[1] allows efficient excitonic emission evenat room temperature. In addition, ZnO doped with transi-tion metals shows great promise for spintronic applica-tions.[3] It has also been suggested that ZnO exhibits sensi-tivity to various gas species, namely ethanol (C2H5OH), ace-tylene (C2H2), and carbon monoxide (CO), which makes itsuitable for sensing applications. Moreover, its piezoelectricproperty (originating from its non-centrosymmetric struc-ture) makes it suitable for electromechanical sensor or ac-tuator applications. Also, ZnO is biocompatible whichmakes it suitable for biomedical applications. Last but notleast, ZnO is a chemically stable and environmentallyfriendly material. Consequently, there is considerable inter-est in studying ZnO in the form of powders, single crystals,thin films, or nanostructures.

A variety of ZnO nanostructure morphologies, such asnanowires,[4–9] nanorods,[10–14] tetrapods,[14–18] and nanorib-bons/belts,[6, 18–21] have been reported. ZnO nanostructureshave been fabricated by various methods, such as thermalevaporation,[4–6,9, 14–21] metal–organic vapor phase epitaxy(MOVPE),[12] laser ablation,[13] hydrothermal synthesis,[7, 10,11]

and template-based synthesis.[8] Recently, novel morpholo-gies such as hierarchical nanostructures,[22] bridge-/nail-likenanostructures,[23] tubular nanostructures,[24] nanosheets,[25]

nanopropeller arrays,[26,27] nanohelixes,[26,28] and nano-ACHTUNGTRENNUNGrings[26,28] have, amongst others, been demonstrated. Someof the possible ZnO nanostructure morphologies are shown

in Figures 1–3. Several recent review articles have summar-ized progress in the growth and applications of ZnO nano-structures.[29–31] The growth and properties of ZnO nano-structures have been extensively studied,[32–75] but there arestill a number of unanswered questions concerning the rela-tionship between fabrication conditions and optical proper-ties.

The fabrication methods for ZnO nanostructures can bedivided into two groups: spontaneous growth and template-based synthesis (for example, using an alumina template).Fabrication without a template can occur either by usingmetal catalysts or may be self-catalyzed. The use of metalcatalysts, such as Au, can be an advantage for achievingaligned and selective area growth.[4] Aligned nanorods canalso be obtained by a hydrothermal method without anymetal catalyst.[7,46] The degree of alignment and the ach-ieved aspect ratio was dependent on the seed layer usedand the fabrication conditions.[7,46] An improvement inalignment of the rods perpendicular to the substrate was ob-tained when zinc acetate was used to prepare the nanocrys-talline seed layer instead of ZnO nanoparticles.[7] Thegrowth of ZnO by vapor deposition is typically affected bytemperatures of the source and the substrate, the distancebetween the source and the substrate, the heating rate, thegas flow rate, tube diameter, and the starting precursor(s).[75]

The influence of these factors on ZnO morphology was

[*] Dr. A. B. DjurisicDepartment of Physics, The University of Hong KongPokfulam Road (Hong Kong)Fax: (+852)2559-9152E-mail: [email protected]

Y. H. LeungDepartment of Chemistry, The University of Hong KongPokfulam Road (Hong Kong)

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Optical Properties of ZnO Nanostructures

studied in detail recently,[75] but the effects of these factorson the optical properties of fabricated nanostructures arestill unknown. Different experimental conditions, such as

for example, solution concentration, temperature, and sub-strate pretreatment, also affect the growth of ZnO by hy-drothermal methods. Due to the low growth temperature(typically under 100 8C), the crystalline quality of such sam-ples is often lower than those fabricated by vapor deposi-

Figure 1. a–f) Representative scanning electron microscopy images ofvarious ZnO nanostructure morphologies.

Figure 2. a–d) Representative scanning electron microscopy imagesof ZnO nanopropeller arrays. Reprinted with permission fromRef. [27].

Figure 3. A–D) Representative scanning electron microscopy imagesof ZnO helical nanobelts. Reprinted with permission from Ref. [28].

Aleksandra B. Djurisic obtained her PhDdegree in electrical engineering from theSchool of Electrical Engineering at theUniversity of Belgrade (now Serbia) in1997. After finishing her PhD studies,she worked as a postdoctoral fellow atUniversity of Hong Kong and as anAlexander von Humboldt postdoctoralfellow at TU Dresden. She has been as-sistant professor in the Dept. of Physicsat the University of Hong Kong since2003. Her research interests include theoptical properties of materials, nano-ACHTUNGTRENNUNGmaterials, wide-bandgap semiconduc-tors, block copolymers, and opto-ACHTUNGTRENNUNGelectronic devices.

Y. H. Leung obtained his B.Eng. degreefrom the City University of Hong Kong in2003 and his M. Phil. degree from theUniversity of Hong Kong in 2005. He iscurrently working as a research assistantat the University of Hong Kong. His re-search interests include the fabrication,characterization, and applications of ZnOnanostructures.




tion. However, the optical quality of the samples can be im-proved by annealing under appropriate conditions.[46]

In this Review, we provide a detailed overview on theoptical properties of ZnO nanostructures. The Review is or-ganized as follows: In the next section, we discuss spontane-ous emission from ZnO nanostructures. Low-temperatureand room-temperature photoluminescence in the UV andvisible spectral regions are discussed. Next, an overview ofstimulated emission in various ZnO nanostructures is pro-vided. In chapter 4, the nonlinear optical properties of ZnOare presented. Finally, some conclusions and an outlook forthe future are given.

2. Spontaneous Emission

Optical properties of a variety of forms of ZnO, includ-ing ZnO nanostructures, have been studied by photolumi-nescence (PL) spectroscopy.[32–122] The majority of the re-ported luminescence spectra of ZnO nanostructures havebeen measured at room temperature, although variable-tem-perature photoluminescence studies have been performedon some of the samples.[32–46] Room-temperature PL spectraof ZnO typically consist of a UV emission and possibly oneor more visible bands due to defects and/or impurities.

2.1. UV Emission

Low-temperature photoluminescence measurements ofdifferent nanostructures, such as nanowire/nanowall sys-tems,[32] nanosheets,[33] nanowalls,[44] nanowires,[34, 43,45] nano-rods,[35,37,39,46] faceted nanorods,[41] nanoparticles,[42] andnanoblades and nanoflowers,[40] have been reported. Low-temperature (4–10 K) PL spectra of ZnO typically exhibitseveral peaks (labeled I0–I11), which correspond to boundexcitons.[76] An example of a low-temperature PL spectrumof a ZnO sample exhibiting a number of bound-excitonpeaks is shown in Figure 4. The number of observed bound-exciton peaks in ZnO nanostructures is typically lower thanthat in ZnO single crystals. An example of a low-tempera-

ture PL spectrum of highly faceted ZnO rods with goodcrystalline quality[41] is shown in Figure 5. Since the relativeintensity of the bound-exciton peaks varies from sample tosample due to variations in donor/acceptor concentrations

and their capture cross sections,[77] variable-temperature PLmeasurements can provide useful information about the op-tical and structural properties of ZnO. However, the assign-ment of the bound-exciton peaks in ZnO is, in general, con-troversial for all forms of the samples, namely, ZnO singlecrystals, epitaxial films, and nanostructures. For example, itwas proposed that the emission lines I5 to I11 in the lowerpart of the energy spectrum can be attributed to excitonsbound to neutral acceptors.[78] However, other reports in theliterature attributed some of these lines to donor bound ex-citons.[76, 79] The chemical identity of the donors and accept-ors responsible for different bound-exciton lines still re-mains unclear (for a complete list of the bound-excitonpeaks generally observed in ZnO, and a summary of thepossible identification of the donors and acceptors, seeRefs. [1, 76]).

One of the commonly observed bound-exciton lines inZnO nanostructures is the I4 line at �3.3628 eV.[41,82] Thisemission is typically attributed to the donor bound exciton,and the donor has been identified as hydrogen.[76, 80,81] Theo-retical calculations predict hydrogen to be a shallow donorin ZnO[123] and it is reasonable to expect that an uninten-tional incorporation of hydrogen could frequently happen inZnO nanostructure synthesis. While in general there is aconsensus in assigning the I4 line to hydrogen donors,[76,80,81]

the chemical identity of donors responsible for other donorbound-exciton lines remains unclear.

For the acceptor bound excitons, the most commonly re-ported peak is located at 3.3564 eV.[77] This peak is common-ly attributed to excitons bound to Na or Li acceptors.[1]

Alkali metals are predicted to produce shallow acceptors onthe cation site, but the experimental results demonstratethat doping with group 1 ions produces complex results.[124]

Figure 4. Bound-excitonic region of the 10 K PL spectrum for theforming gas annealed ZnO substrate. Reprinted with permission fromRef. [77].

Figure 5. PL spectra from ZnO faceted rods at different temperaturesas a function of a) wavelength, and b) energy. The inset shows anenlarged region of the PL spectrum at 7 K. Reproduced fromRef. [41].

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However, other acceptor levels have also been proposed,such as an acceptor complex involving a N impurity on anO site.[81] However, some authors attribute this line to adonor bound exciton instead.[76, 79] Bound-exciton lines I6, I8,and I9 have been assigned to excitons bound to Al, Ga, andIn donors, respectively.[76] On the other hand, Thonkeet al.[82] proposed that the weak 3.357 eV line correspondsto the acceptor bound exciton, while the I8 line at 3.3597 eVwas found to be a donor bound-exciton line.

In addition to commonly observed acceptor bound-exci-ton lines, emission at 3.332 eV (labeled as Ia) was recentlyobserved in low-temperature PL spectra of ZnO epilayersgrown on CaF2 ACHTUNGTRENNUNG(111).[109] Since this peak occurs in the spec-tral region where two-electron satellites (TES) of donorbound-exciton peaks are expected to occur,[82] careful ex-amination of the peak position in respect to known bound-exciton positions and expected TES peaks (see Ref. [76] forthe positions of TES lines for different bound-excitonpeaks) is necessary. In addition, the occurrence of peak near3.333 eV may indicate excitons bound to structural de-fects.[76] Therefore, further work is needed for conclusiveidentification of the origin of different bound-exciton linesin ZnO. The assignment of several bound-exciton lines, es-pecially I9, is still controversial and conclusive chemicalidentification of the majority of donors and acceptors hasnot been accomplished.

At low temperatures, in addition to bound-excitonpeaks, two-electron satellite transitions can be observed inthe spectral region 3.32–3.34 eV.[77] These transitions corre-spond to a radiative recombination of donor bound excitons,which leaves the donor in an excited state. Thus, they are lo-cated at an energy lower by an amount equal to the differ-ence between the first excited and ground states of thedonor, so that their position in respect to the donor bound-exciton peaks can be used to estimate donor-binding ener-gies.[82,83]

Finally, low-temperature PL spectra can also containdonor–acceptor pair transitions and longitudinal optical(LO) phonon replicas.[77] First-, second-, and third-order LOphonon replicas can typically be observed.[44] The LOphonon energy can be determined from the separation be-tween the exciton peaks and their LO phonon replicas, andfor ZnO it is 71–73 meV.[45,84] Since donor–acceptor pairtransitions and some of the LO phonon replicas occur in thesame spectral region (3.218–3.223 eV),[77] care needs to betaken in assigning the peaks observed in this region. In addi-tion, two-phonon replicas due to two transverse optical pho-nons (separation of �108 meV) were also reported in ZnOthin films prepared by spray pyrolysis.[113]

With regard to the temperature dependence of the ob-served peaks, a red shift of the free-exciton emission withincreasing temperature occurs. The intensity of the bound-exciton peaks and the LO phonon replicas decreases withincreasing temperature, and only free-exciton emission canbe observed at room temperature. In ZnO epilayers, free-exciton emission was found to dominate the spectra above�70 K.[109] Similar behavior, with the disappearance ofbound-exciton peaks above 150 K, was also observed inZnO single-crystal samples.[110] The bound-exciton line for

ZnO nanoparticles embedded in alkali halide crystals alsodisappeared at �125 K.[115] The exact temperature at whichthe bound-exciton line will disappear depends on the identi-ty of the donors or acceptors, since different donors/accept-ors will be thermally ionized at different temperatures. Itshould be noted that in the case of donor–acceptor pairtransition, disappearance of this peak with increasing tem-perature can be accompanied by the appearance of acceptorbound-exciton peaks if the acceptors are thermally ionizedat higher temperature than the donors.[117]

However, what all ZnO samples (single crystals, films,and nanostructures) have in common is the disappearanceof bound-exciton peaks at temperatures in the range 50–150 K, while at room temperature only free-exciton emis-sion is observed. The presence of free-exciton emission atlow temperatures, as well as a distinction between A and Bexciton peaks, is usually considered to indicate high qualityin ZnO samples.[111] It should be noted that this criterion forsample quality is less arbitrary than the ratio between UVand defect emission, which is sometimes used to estimatesample quality,[116, 121] and which is dependent on excitationarea and power.[74] Distinction between A and B excitonpeaks is usually not possible above 160 K,[77] and thephonon replicas typically cannot be clearly resolved above�250 K.[44] In very-high-quality samples, higher-order exci-ton lines can also be observed at low temperatures.[118,122]

Biexciton emission was observed at 77 K in high-qualityepitaxial ZnO films.[111] The biexciton binding energy wasestimated to be 15 meV.[111] Biexciton emission has alsobeen observed in ZnO nanowires[112] and nanorods.[35,114]

The obtained energy separation between exciton and biexci-ton peaks in ZnO nanowires was 20 meV, in good agree-ment with the results obtained on other forms of ZnO.[112]

The reported energy separation in the case of nanorods was18 meV,[35] and biexciton emission persisted up to�200 K.[114] Clear observation of free-exciton and biexcitonlines at low temperatures is usually considered as an indica-tion of very good sample quality.[35]

In room-temperature PL spectra, some variation of theposition of the PL peak can be observed for different nano-structures. This is illustrated in Figure 6, where different UV

Figure 6. Room-temperature PL spectra of various nanostructures inthe UV range: 1) Tetrapods, 2) needles, 3) nanorods, 4) shells,5) highly faceted rods, 6) ribbons/combs.




peak positions (387 nm for tetrapods, 381 nm for needles,397 nm for nanorods, 377 nm for shells, 379 nm for facetedrods, and 385.5 nm for ribbons/combs) can be observed.Room-temperature band-edge emission in ZnO nanostruc-tures was reported to occur at 373,[49] 378,[46,53,57] 380,[47, 64,67]

381,[55] 383,[52, 62] 384–391,[56] 387.5,[58] 389,[50, 62] and 390 nm.[59]

Individual nanostructures, such as nanobelts, exhibited UVemission in a range between 384 and 391 nm.[56] These dif-ferences in the peak positions of individual nanobelts, whichare sufficiently large so that there could be no quantum con-finement effects, indicate that there is likely a different ex-planation for the variation in the band-edge emission inZnO nanostructures reported in different studies. Eventhough quantum confinement has been proposed as a causeof the blue shift of the band-edge emission with decreasingsize,[69] any shift due to quantum confinement in nanocrys-tals with diameters of 57, 38, and 24 nm is not likely consid-ering the fact that the Bohr radius of ZnO is 2.34 nm.[70]

One possible reason for the variations in the position ofthe band-edge emission in various ZnO nanostructures withrelatively large dimensions are different concentrations ofnative defects. Since the defect density on the surface ishigher than in the bulk,[125] spectral shifts due to differentdefect concentrations are expected to occur in nanostruc-tures with different sizes due to different surface-to-volumeratios. The fact that the decay times in time-resolved PLfrom ZnO nanorods are size dependent[71] is in agreementwith the assumption of different defect levels/concentrationsfor structures with different surface-to-volume ratios. Thus,the defects could affect the position of the band-edge emis-sion as well as the shape of the luminescence spectrum. Al-though there have been several reports with strong UV andweak defect emission in ZnO nanostructures,[49,50] in somecases only defect emission is observed[48] or the UV emissionis much weaker compared to the defect emission.[46] There-fore, clarifying the origins of different defect emissions is animportant issue. However, it should be noted that the ratioof the intensity of UV and defect emission is dependent onthe excitation density,[36,74] as well as the excitation area.[74]

Thus, the ratios of these two emissions cannot be used as anabsolute determining factor of the crystalline quality ofZnO, although they are useful in comparing the quality ofdifferent samples when the measurements are performedunder identical excitation conditions.

2.2. Defect Emissions

Room-temperature PL spectra from ZnO can exhibit anumber of different peaks in the visible spectral region,which have been attributed to the defect emission. Emissionlines at 405, 420, 446, 466, 485, 510, 544, 583, and 640 nmhave been reported (see Ref. [36] and references therein).Several calculations of the native defect levels in ZnO havebeen reported,[85–87, 124] as summarized in Figure 7. An exam-ple of defect emissions (normalized PL spectra) from differ-ent ZnO nanostructures is shown in Figure 8.

Green emission is the most commonly observed defectemission in ZnO nanostructures,[4,47–49,52,53,56–58,61,62,64,67,68]

similar to other forms of ZnO. The intensity of the blue–green defect emission was found to be dependent on thenanowire diameter,[5,64] but both increased[5] and de-creased[64] defect emission intensity with decreased wire di-ameter were reported. Several different hypotheses havebeen proposed: Green emission is often attributed to singlyionized oxygen vacancies,[47–49, 64,68] although this assignmentis highly controversial. Other hypotheses include antisiteoxygen,[56] which was proposed by Lin et al.[85] based on theband structure calculations. Green emission was also attrib-uted to oxygen vacancies and zinc interstitials.[67] Cu impuri-ties have been proposed as origin of the green emission inZnO.[88] Blue-green defect emission was also reported in Cudoped ZnO nanowires.[65] However, although Cu was identi-fied as a possible cause of green emission in ZnO,[88] thiscannot explain the defect emission in all ZnO nanostructuresamples, especially those where defect emission exhibitsstrong dependence on annealing temperature and atmos-phere which would be more consistent with an intrinsicdefect rather than Cu impurity. Other hypotheses include

Figure 8. Room-temperature PL spectra of different nanostructures:1) Tetrapods, 2) needles, 3) nanorods, 4) shells, 5) highly facetedrods, 6) ribbons/combs.

Figure 7. Illustration of the calculated defect energy levels in ZnOfrom different literature sources (data marked with the subscript “a”originate from Ref. [85], those marked with “b” stem from Ref. [87],and those marked with “c” originate from Ref. [86]). VZn, VZn

� , andVZn

2� denote neutral, singly charged, and doubly charged zinc vacan-cies, respectively. Zni

o and Zni indicate neutral zinc interstitials,while Zni

+ denotes a singly charged zinc interstial. VOo and VO denote

neutral oxygen vacancies, while VO+ denotes a singly charged

oxygen vacancy. Oi represents an oxygen interstitial. VOZni denotes acomplex of an oxygen vacancy and zinc interstitial.




various transitions related to intrinsic defects, such asdonor–acceptor transitions,[89] recombination at Vo** centers(where these centers are generated by surface trapping ofphotogenerated holes, followed by recombination with elec-tron in an oxygen vacancy Vo*),[90, 91] zinc vacancy,[92, 93] andsurface defects.[36] Although the singly ionized oxygen va-cancy[94] is a commonly cited hypothesis, which is supportedby reports of the enhancement of the green defect by an-nealing at temperatures above 600 8C (attributed to out-dif-fusion of O),[47] this assignment has been questioned recent-ly.[36, 88] The donor–acceptor transition hypothesis used to ex-plain the green and yellow emissions has also been chal-lenged.[95] On the other hand, while the Zn vacancy hypoth-esis is supported by the study of the effect of O and Znimplantation,[93] a blue rather than green emission would beexpected based purely on the theoretically predicted energylevels for Zn vacancy.[86] Therefore, the origin of the greenemission is still an open and controversial question and theidentification of the exact origin of this emission requiresfurther study.

While the type of defect responsible for the green emis-sion has not yet been conclusively identified, there is con-vincing evidence that it is located at the surface. It wasshown that coating ZnO nanostructures with a surfactantsuppressed green emission.[36] Polarized luminescence ex-periments from aligned ZnO nanorods also indicated thatgreen emission originated from the surface of the nano-rods.[72] The surface recombination layer responsible for visi-ble emission in ZnO nanowires was estimated to be �30 nmin thickness.[73] Also, the possible presence of Zn(OH)2 atthe surface, especially for nanostructures prepared by solu-tion methods, could affect the emission spectra from ZnOnanostructures.[51]

Yellow defect emission is also commonly reported inZnO nanostructures,[46, 62,96] and it represents a common fea-ture in samples prepared from aqueous solutions of zinc ni-trate hydrate and hexamethylenetetramine.[46,96] This emis-sion is typically attributed to an oxygen interstitial,[46,92,96] al-though a Li impurity represents another possible candi-date.[96] The deep levels responsible for green and yellowemissions were found to be different;[92, 96] unlike the defectresponsible for the green emission, the defect responsiblefor the yellow emission is not located at the surface.[96]

In addition to green and yellow emissions, orange-redemissions are often also observed.[53,57,67,97,98] Fan et al.[53,57]

reported that the visible emission in ZnO dendritic wiresand nanosheets consisted of two components centered at�540 and �610 nm. The intense visible emission in ZnOnanosheets was tentatively attributed to surface disloca-tions.[57] Orange-red emission at �626 nm in ZnO nanorodswas attributed to oxygen interstitials.[67] In addition, orangeemission at �640–650 nm in ZnO needles[98] and nano-wires[97] was proposed to be due to oxygen-rich samples, inagreement with a previous study on ZnO films.[99] This emis-sion could be reduced by annealing under vacuum or in aH2/Ar mixture.[97] However, although these treatmentsquenched the visible defect emission, near-infrared (NIR)emission at �750 nm was enhanced.[97] It was shown thatgreen, yellow, and red-NIR emissions originate from differ-

ent types of defects by depth-resolved cathodoluminescenceand PL measurements.[100] The NIR and the yellow emis-sions were found to have different decay properties, and itwas proposed that they involved a similar final state relatedto excess oxygen but with different initial states (conductionband and donor centers).[101] It should be noted that al-though the majority of studies attribute red-NIR emissionto excess oxygen, zinc interstitials were also proposed to ex-plain the origin of a red emission in ZnO particles.[102] Thus,although this emission is less controversial than the greenone, further studies are needed to clarify its origin.

A similar conclusion applies to other defect emissionsreported in ZnO, such as blue and violet defect emissions.Zhao et al.[55] reported emission at 3.0 eV (413 nm), whichwas attributed to a zinc vacancy, while a violet emission inZnO nanobelts at �421 nm was attributed to interstitialzinc.[56] Blue emission at �440 nm was reported for tetrapo-dal nanocrystals,[59] while other reports indicate the presenceof both violet (419 nm) and blue (438 nm) emissions in ZnOtetrapods.[60] The violet emission was attributed to interfacetraps, while blue emission was attributed to oxygen vacan-cies.[60] A blue emission band (�420 and 444 nm) in ZnOnanowires[63] and ZnO nanocrystals (at �442 nm)[68] wasalso attributed to oxygen vacancies. It is obvious that formany types of defects, different studies assign them differentemission-peak positions. Thus, while the origins of some ofthe emission peaks (such as yellow for example) are lessdoubtful, the origin of defect emissions in ZnO is still an un-resolved question in spite of a large number of reports.

In addition to identifying the origin of the defect emis-sions, an important question is the suppression of defectemission either by varying the fabrication conditions or bypost-fabrication treatment. It was reported that the greenemission from ZnO nanoparticles can be suppressed by em-bedding the nanoparticles into a synthetic opal whose pho-tonic bandgap overlaps with the deep-level emission.[61] An-other way to suppress green defect emission is by coating ofthe surface with surfactant.[36] Hydrogen plasma was alsoshown to enhance UV-to-defect emission intensity ratio forZnO nanorods.[103] As for the yellow emission, it has beenshown that it can be reduced by annealing in a reducing en-vironment (hydrogen/argon mixture).[46]

2.3. Defect Identification using Other Techniques

There are several techniques that can be used combinedwith photoluminescence to identify the origin of the defectemissions in ZnO. A useful technique for identifying para-magnetic defects is electron paramagnetic resonance (EPR)spectroscopy, which has been used to study defects inZnO.[36,87,93,95,104,126] For an overview of previous studies seeRef. [126], while some of the more recent results are givenin Ref. [36]. Similar to the origin of the green luminescence,the assignment of the peaks in EPR spectra of ZnO hasbeen controversial. The singly ionized oxygen vacancy hy-pothesis was proposed by Vanheusden et al.[92] based on thecorrelation between the EPR peak at g�1.96 and greenemission intensity. However, other studies attribute the




peak at g�1.99 to oxygen vacancies.[88] Although the assign-ment of the peaks is in question, this technique is still auseful tool to obtain more information about the defects,and its application to nanostructures is straightforward sincenanostructures can be treated as any other sample in pow-dered form.

Positron annihilation spectroscopy (PAS) has been usedin the past together with photoluminescence to study defectemissions in ZnO.[105, 106] Although no conclusive identifica-tion of the defects responsible for the visible emissions hasbeen made, this technique still enables more information tobe obtainined about the defect levels, which helps to at leasteliminate some of the possible candidates for the causes ofdefect emission. It should be noted however that althoughPAS has been applied to some nanostructured samples, theinterpretation of the data from nanorods or nanowireswould be very complicated due to the voids in the samples.Another useful technique for studying defect levels is deep-level transient spectroscopy (DLTS). A DLTS study of ZnOsingle crystals established that defects at 0.10, 0.12, 0.29, and0.59 eV below the conduction band were found,[107] althoughthe defect identity was not conclusively established. Again,this technique is also more applicable to thin-film or single-crystal samples. However, it is expected that conclusionsfrom studying defect emission from ZnO in these formscould be extrapolated to identify defects in nanostructuressince defect positions and behavior are similar for nano-structured and bulk samples.

3. Stimulated Emission

Due to its high exciton-binding energy, ZnO is of inter-est for the achievement of excitonic stimulated emission atroom temperature, which has a lower threshold than elec-tron–hole plasma recombination. While there have been nu-merous reports on optically pumped lasing and amplifiedspontaneous emission from ZnO,[4,41,98,127–168] no electricallypumped lasing has been achieved as yet. Amplified sponta-neous emission was reported for a self-organized network ofZnO fibers,[161] while lasing has been reported in a numberof different structures such as, for example, nanowires,[5,138]

tetrapods,[151–154] and nanoribbons/combs.[154] A very broadrange of lasing thresholds has been reported for differentZnO nanostructures, ranging from 8 kWcm�2 (ZnOfibers)[130] to 867 kWcm�2 (ZnO nanorods).[163] In the follow-ing section, the experimental results of the stimulated emis-sion in ZnO nanostructures will be summarized. The discus-sion of the basic principles will include some comparisonswith stimulated emission from other forms of ZnO.

3.1. Gain Mechanism

Stimulated emission in ZnO can be achieved either byexciton–exciton (EE) scattering or electron–hole plasma(EHP) recombination. As the excitation power increases,sharp peaks will appear in the emission spectra from ZnO(highly-faceted rods), as illustrated in Figure 9. Due to the

significantly shorter decay time of the stimulated emissioncompared to spontaneous emission, lasing peaks can be ob-served more clearly in time-resolved spectra, as shown inFigure 10. As the excitation power increases, the increase inintensity and the appearance of narrow lasing modes can beobserved. With a further increase of excitation power, lasingin the EHP mode occurs. The lasing in these different exci-tation regimes will be discussed below.

3.1.1. Exciton–Exciton Scattering

The peak position of the emission resulting from inelas-tic collisions between excitons is given by:[167]

En ¼ Eex � Ebex 1 � 1

�n2

� �� 3kT=2 ð1Þ

where n=2, 3,…, k is the Boltzmann constant, T is the tem-perature, and Eb

ex =60 meV is the exciton binding energy.

Figure 9. Emission spectra from highly faceted ZnO rods at differentexcitation powers. The excitation wavelength was 267 nm, and thepulse duration was 1 ps.

Figure 10. Time-resolved PL spectra for spontaneous emission(shown at 4 ps), stimulated emission due to exciton–exciton scatter-ing (shown at 8 ps due to a longer delay time), and stimulated emis-sion due to EHP (shown at 4 ps). Due to the very high intensity emis-sion in the EHP regime, the other two spectra have been multipliedby a factor of 10 to improve the clarity of presentation. Reprintedfrom Ref. [41].




The transition from spontaneous emission to stimulatedemission is evidenced by narrowing of the emission with afull-width half maximum (FWHM) about two orders ofmagnitude lower compared to the FWHM of spontaneousemission.[156] The threshold for lasing due to exciton–excitonscattering in nanostructures is typically 2–3 timeslower[98,138,154] than that for lasing in the EHP regime.

3.1.2. Electron–Hole Plasma

With increasing excitation energy, the density of the ex-citons in ZnO will also increase. As the exciton density in-creases, binding energy decreases. The EHP plasma formsat densities higher than the “Mott density”, given by:[1]

nM ¼ kT2a3

BEbex

ð2Þ

where aB is the Bohr radius. This is estimated to be �3.7K1019 cm�3,[1] although lower estimates such as �4K1018 cm�3

have also been reported.[138] The EHP emission is typicallymore broad and red shifted compared to emission due toexciton–exciton scattering.[1] The red shift of the EHP emis-sion is the result of bandgap renormalization.[140] Coexis-tence of EE and EHP emissions observed in ZnO thin filmswas attributed to spatial nonuniformity of the sample andthe beam profile.[127] However, time-resolved studies on dif-ferent ZnO nanostructures indicate that the coexistencemay originate from the fast decay of EHP emission since ablue shift of the emission with time and eventual bandgaprecovery could be clearly observed in time-resolved spec-tra.[154] The evolution of the lasing spectra with an increaseof excitation power has been studied in detail for singleZnO nanowires.[138]

3.2. Feedback Mechanism

The coherent feedback in ZnO nanostructures or nano-structured films can be provided by two basic mechanisms,as illustrated in Figure 11. In the first case, coherent feed-back is provided by multiple reflections from the end facetsof the nanostructure, which serves as a Fabry–Perot resona-tor. It has been shown that UV emission is typically en-hanced at the ends of the nanowire, while defect (green)emission is emitted from all parts of the nanowire,[138] asshown in Figure 12. In the second case, the coherent feed-back is provided by multiple scattering events. An exampleof random lasing from ZnO is illustrated in Figure 13.Random lasing and individual nanostructures as Fabry–Perot resonators are discussed in detail below.

3.2.1. Random Lasing

In random lasers, coherent feedback is provided by re-curring scattering events.[169] Cavities are “self-formed”, andthe main requirement to observe this type of lasing is thatthe scatterer size is smaller than the emission wave-length.[162] The dependence of the lasing on the excitation

area has been demonstrated in ZnO polycrystalline films,[128]

nanorods,[133] and nanowires,[145] and the closed loops alongwhich lasing occurred have been observed.[128] Random

Figure 11. Illustration of two different feedback mechanisms:a) Nanostructures as Fabry–Perot resonators, b) a random laser.

Figure 12. Images of green/UV PL: a) Topographic, b) UV, andc) green near-field PL images. Scale bar=5 mm. d) Far-field image ofgreen/UV PL showing enhancement of the UV PL near the end of thewire. Scale bar=5 mm. Reprinted with permission from Ref. [138].




lasing in ZnO was reported in polycrystalline thinfilms,[128,143,157,159] nanoparticles,[155, 160] ZnO powder films,[162]

nanorods,[133,143] nanoneedles,[156] and nanowires.[145] The filmstructure and crystallinity were found to affect the lasingthreshold,[143] and the lasing characteristics were also foundto be dependent on the strain.[157] Random lasing was alsodemonstrated from thin-film ZnO ridge waveguides, with[159]

and without[159] a MgO capping layer. The lasing thresholdin ZnO nanoneedle random lasers was found to be signifi-cantly lower than the reported value for conventional lasingin ZnO nanocolumns.[155]

One distinguishing characteristic of a random laser isthat stimulated emission can be observed in all directions,and the mode structure in the measured spectra shows angu-lar dependence.[162] While the interpretation of the data isstraightforward in the case of polycrystalline films, for thecase of a nanowire ensemble with random nanowire orienta-tion, it is difficult to establish from angular-dependencemeasurements of the emission spectra whether the feedbackoriginates from individual wires acting as resonators or fromscattering between the wires. Another distinguishing charac-teristic of a random laser is that the lasing threshold de-pends on the excitation area.[162] The lasing threshold in-creases as the excitation area decreases, and eventually nolasing can be observed in areas smaller than a criticalsize.[162] However, careful analysis of the obtained data isneeded when measurements are performed on an ensembleof nanostructures instead of polycrystalline films. Since dif-

ferent nanostructures canhave different lasing thresh-olds,[138] reduction in a lasingthreshold observed with in-creased excitation area couldsimply be a result of an indi-vidual nanostructure with alower threshold becomingexcited.

One possible way to dis-tinguish between the twopossible feedback mecha-nisms is to analyze the Fouri-er transform of the lasingspectrum.[145] In order to dis-tinguish between the possi-bility of having multipleclosed loops and multiplelasing modes in individualFabry–Perot cavities formedby crystal facets of the nano-structrures, it may be neces-sary to observe where thelasing originates from, as de-scribed elsewhere.[128] Carefulinterpretation of the lasingdata is necessary to conclu-sively establish the feedbackmechanism. Althoughrandom lasing has been pro-posed to explain stimulated

emission from short needles with sharp top surfaces,[156]

stimulated emission was observed from individual, relativelyshort nanorods with pyramidal tops indicating that multiplescattering was not necessary to achieve stimulated emis-sion.[166]

3.2.2. Nanostructures as Fabry–Perot Resonators

Lasing from ZnO nanowires with each wire forming aFabry–Perot resonator bound by reflecting (0001) facets wasfirst reported by Huang et al.[4] Since then, there have beennumerous reports on stimulated emission from various ZnOnanostructures. Lasing was reported in microtubes,[129,150]

nanocoral reefs and nanofibers,[130] whiskers,[131] nano-wires,[4,138,139,147–149] nanorods,[132,134,154] nanoribbons,[135, 136,139]

nanocombs,[137,154] and tetrapod nanostructures.[151–154,168]

While some of the measurements have been performed onnanostructure ensembles, stimulated emission from individ-ual nanostructures[135,136,138,139,151,152] as well as nanostructuresdispersed with very low density (<10 per laser spot)[168] wasalso obtained, clearly demonstrating that in those cases thefeedback could not be obtained from multiple random scat-tering. While in some cases, such as individual nanowires[138]

and nanoribbons,[138,139] the identification of the cavity isstraightforward, lasing has also been demonstrated withnanostructures with more complex morphologies. Also,stimulated emission was obtained not only for nanostruc-tures fabricated at high temperatures by vapor deposi-

Figure 13. a) Schematic diagram of the laser measurement setup. b) Light curves for the samples aftervarious ion-irradiation times. The inset shows the maximum emission intensity of the TE mode as a func-tion of polarization angle. c) Emission spectrum of the as-grown ZnO thin film under a pump power of1.6 MWcm�2. d) Evolution of emission spectra of the irradiated sample (30 min) under different pumpintensities. e) Emission spectrum of the sample irradiated for 60 min under a pump power of1.6 MWcm�2. Reprinted with permission from Ref. [156].




tion,[131, 137,154,168] but also for nano-structures fabricated at low tem-perature from aqueous solu-tion.[134, 154]

Detailed mode analysis ofnanostructures as laser cavities hasbeen performed.[138, 153] Goodagreement in ZnO nanowires be-tween the measured modes withsimilar polarization and expectedmode spacing is given by:[138]

Dl ¼ l2

2L n� ldndl

� � ð3Þ

where Dl is the mode spacing, l isthe wavelength, n is the refractiveindex, and L is the length. Thelinewidth of the measured lasingmodes was also in good agreementfrom the theoretical estimate ofthe order of 1 nm obtained for aFabry–Perot resonator by using thefollowing expression:[138]

Dn ¼ � c4pLn

ln R1R2 1 � Tið Þ2� �

ð4Þ

where c is the speed of light, R1

and R2 are the reflectivities of themirrors, and Ti is the transmittanceof the internal medium of thecavity. The threshold gain can beexpressed as:[135]

gth ¼ aþ 12L

ln�1 R1R2ð Þ ð5Þ

where L is the length and a is the absorption loss. It shouldbe noted, however, that thresholds in individual nanowirescan vary by orders of magnitude, which was attributed todifferences in dimensions, the condition of the cavity, andthe extent of substrate coupling.[138] The threshold may alsobe affected by damage of the nanowire during transfer fromthe growth substrate to the support substrate for the meas-urements.[153]

Among other more complicated cavities, ZnO tetrapodshave been most thoroughly studied.[151–154, 168] The lasingthreshold and mode structure of a ZnO tetrapod werefound to be dependent on the tetrapod position on the sub-strate (resting on one arm or three arms), which indicatedthat the substrate coupling and the collection geometryplayed a role in the lasing spectra.[153] Intercavity couplingbetween different arms was also identified as a possiblefactor affecting the lasing threshold and the mode structure,although this effect is likely not very significant since re-moval of one tetrapod arm had only a small effect on thelasing properties, as shown in Figure 14.[153] This is in agree-ment with other reports which have shown that each leg of

the tetrapod acts as an individual laser.[151, 152] The lasingfrom multiple tetrapod arms occurs only if all the arms areexcited, otherwise lasing will occur only from the excitedarm due to high losses in the non-excited tetrapod arms.[152]

3.3. Factors Affecting the Achievement of StimulatedEmission

It has been pointed out that very high lasing thresholdsare obtained in ZnO films with poor crystallinity.[143] In-crease in the threshold with a decrease in crystal quality wasattributed to an increased concentration of nonradiative de-fects.[143] On the other hand, lasing has been successfully ob-tained from nanorods grown by hydrothermal meth-ods,[134,154] which typically have inferior crystallinity to sam-ples fabricated by vapor deposition due to the low synthesistemperature. The presence of strong defect emission doesnot prevent stimulated emission, since the UV-to-visibleemission intensity ratio increases with increasing excitationpower.[36] Also, the long decay time of spontaneous emis-sion, which indicates excellent crystal quality, is not necessa-

Figure 14. ZnO tetrapod manipulation and lasing in various configurations. a) Spectra recorded at91, 284, 468, and 738 mJ cm�2, showing stimulated emission in the three-arms-down configura-tion. Inset (left): Power dependence curve showing a lasing threshold of �430 mJ cm�2. Inset(right): PL image of the lasing tetrapod depicting the vertical arm as a bright spot in the middle ofthe structure. Scale bar=5 mm. b) Spectra recorded at 255, 454, 596, and 766 mJ cm�2 after flip-ping the tetrapod into the three-arms-up configuration. The lasing threshold for this geometryincreased to �520 mJ cm�2 and the mode shape was drastically altered in comparison to (a).Inset: PL image of the lasing tetrapod. Scale bar=5 mm. c) SEM image of the same tetrapod in(a), (b), and (d) after removing one of the arms with a micromanipulator. Scale bar=1 mm.d) Lasing spectra of the three-armed “tetrapod” shown in (c). The mode structure is similar to (b),but the lasing threshold increases further to �584 mJ cm�2. Inset: PL image of the lasing tripodshowing larger scattering loss from the left (top arm in the SEM image) damaged arm. Scalebar=5 mm. Reprinted with permission.[153]




ry for the achievement of lasing. Biexponential decay withtime constants of 70 and 350 ps was reported for ZnO nano-wires with a lasing threshold of 40 kWcm�2.[4] On the otherhand, highly faceted rods with long luminescence decaytimes (116 ps and 1.2 ns) exhibited a lasing threshold of�45 mJcm�2 or 150 MWcm�2.[41]

It has been shown that stimulated emission can be ach-ieved in nanostructures with different decay times and dif-ferent defect emissions, but it could not be achieved if poorcrystal quality and high cavity losses occur simultaneous-ly.[170] The lasing threshold will therefore be mainly depend-ent on the dimensions of the nanostructure (see [Eq. (5)]),quality of the cavity, and experimental conditions. Also, thelasing thresholds obtained for different excitation wave-lengths should not be directly compared due to different ab-sorption of ZnO at different wavelengths. Due to great var-iations in the lasing thresholds of individual nanowires[138]

and different experimental conditions, it is difficult to com-pare the lasing thresholds from different reports in the liter-ature. Sometimes the reported results are contradictory. Forexample, it was reported that nanoribbons had a higherthreshold compared to tetrapods and nanowires,[153] whilenanoribbon/nanocomb mixtures synthesized by a differentmethod exhibited a lower threshold than tetrapods andnanorods.[154] However, the fact that lasing was demonstrat-ed in a great variety of nanostructure morphologies indi-cates that optically excited stimulated emission is easy toachieve in the majority of ZnO nanostructures. Improvedcrystallinity and larger dimensions can result in lower lasingthresholds, though some variation among nanostructureswith similar dimensions fabricated in the same depositionprocedure is expected due to different conditions of thecavity.

3.4. Time-Resolved Studies of Stimulated Emission

Time-resolved studies of stimulated emission in ZnOhave been performed by severalgroups.[41,98,127,138–141,151,152,154,163,168] The studies have been per-formed on ZnO thin films,[127,140,141] nanowires,[138, 139] nano-ribbons,[139] highly faceted rods,[41] nanoneedles,[98] nanorib-bons/combs,[154] tetrapods,[151,152,154,168] and nanorods.[154,163]

Typically, the stimulated emission decay time is much fasterthan that of spontaneous emission, so that it may be belowthe detection limit of some time-resolved photolumines-cence systems.[149] Emissions in the EE and EHP regime ex-hibit different behaviors with time.[41,98,154,168] The compari-son between the decay curves of the spontaneous emission,EE, and EHP emissions from highly faceted rods is shownin Figure 15. It is clear that although both types of stimulat-ed emission have a shorter decay time compared to sponta-neous emission, there are obvious differences in their tem-poral evolution. EHP emission typically has a short risetime (1–2 ps), which can be attributed to the thermalizationof the hot carriers.[140,141] The EHP emission peak exhibitssome shifting with time, which was established by directmeasurements of the lasing spectra[98,140,154,168] as well as bymeasuring transient profiles of the lasing dynamics as a

function of wavelength.[152] The decay time of EHP emissionis typically just a few picoseconds.[98,151,152,154,168] It was sug-gested that long cavity length, lower losses at end facets,and lower defect concentrations would result in longerdecay times of the lasing.[152]

Unlike EHP emission, stimulated emission in the EEregime can exhibit a longer delay time before the onset ofthe emission.[41, 139,154,168] This was attributed to the longertime needed to achieve a high concentration of excitons inthe excited state.[139] Similar to EHP emission, decay time isalso typically just a few picoseconds.[41,154,170] With respect tothe evolution of the lasing spectra and any peak shifts in theEE regime, some spectral shifts of the peaks can be ob-served with time,[154] but it is difficult to analyze the data be-cause the measurements have been performed on an ensem-ble of the nanostructures. It should also be noted that as aconsequence of fast decay time of EHP emission and thelonger delay time of EE emission, coexistence of the twocan sometimes be observed in the time-resolved spectra ob-tained at different times.[154]

4. Nonlinear Optical Properties

As a consequence of its non-centrosymmetric crystalstructure, ZnO is expected to have nonzero second-ordersusceptibility. Nonlinear properties have been studied fordifferent forms of ZnO.[171–189] Nonlinear optical response ofC excitons has been measured by the four-wave mixing tech-nique on ZnO single-crystal samples.[174] Second-harmonicgeneration (SHG) was measured in ZnO single crystals,[177]

thin films,[171–174,176,178–185] nanowires,[139,186] and nanorib-bons.[139] SHG in ZnO thin films is dependent on the deposi-tion conditions,[178, 180] film thickness,[171, 181] crystalline struc-ture,[171,179,181] orientation of the crystallites,[182] and the grainshape.[184] The significant part of the SHG signal was foundto be generated at grain boundaries and interfaces.[171] Itwas proposed that the film thickness more significantly af-fects the second-order susceptibilities than the depositiontechnique used.[183] Enhancement of the nonlinear suscepti-

Figure 15. Decay curves for three different emission regimes. Theinset shows an enlarged region in the range from �1 to 30 ps.Reprinted from Ref. [41].




bility compared to the bulk values was obtained for verythin films.[172] The reduction of the second-order susceptibili-ty with increasing film thickness was attributed to thechange in orientation of the polar axis as film thickness in-creases.[183] It was also proposed that under certain experi-mental conditions, third-harmonic generation (THG) com-parable to a conventional SHG signal can be observed inZnO thin films.[186] Sputtered ZnO films exhibited bothsecond- and third-order nonlinear properties in spite of thelack of a preferential growth direction, although the second-order nonlinearities were lower than the bulk values.[173]

However, THG has been less frequently studied[173, 174,186]

than SHG in ZnO.While a transient SHG technique was used to study car-

rier dynamics in ZnO nanoribons and nanowires,[139] andSHG and THG signals were measured in ZnO nano-wires,[186] nonlinear optical properties of other ZnO nano-structures have not been studied. In addition to SHG andTHG studies, two-photon[187,188] and three-photon[187] spec-troscopy and measurements of nonlinear refraction and ab-sorption[188] of ZnO were also reported. Two-photon-in-duced photoluminescence was also reported in ZnO micro-tubes,[189] and two- and three-photon-induced luminescencewas observed in single-crystalline ZnO.[175] However, multi-photon spectroscopy studies, as well as characterization ofthe nonlinear optical properties of nanostructured ZnO ingeneral, have been scarce since both the measurement andthe theory are more complex compared to the characteriza-tion of linear optical properties. Considering the variety ofavailable morphologies and possible fabrication methods forZnO nanostructures, it would be of particular interest ifmore studies of the nonlinear optical properties of differentZnO nanostructures were conducted.

5. Optical Properties of Doped ZnO

The optical properties of doped ZnO have been widelystudied.[190–228] In general, the effect of doping on the opticalproperties can be studied either by examining low-tempera-ture UV spectra for evidence of the appearance of boundexciton peaks not observed in undoped samples, or by ex-amining the visible emission for changes in the defect emis-sion spectra. The doping of ZnO is a research topic of con-siderable interest in its own right, but the discussion will belimited here to the actual doping of nanostructures andnanocrystalline films. There are four main topics of interestin doping ZnO: 1) doping with donor impurities to achievehigh n-type conductivity, 2) doping with acceptor impuritiesto achieve p-type conductivity, 3) doping with rare-earth ele-ments to achieve desired optical properties, and 4) dopingwith transition metals to achieve desired magnetic proper-ties.

While doping to achieve n-type conductivity is straight-forward, the achievement of p-type conductivity is difficultdue to the presence of native defects. In addition to study-ing the effects of impurities on optical, electrical, and mag-netic properties, the effects of different impurities on theorientation of ZnO nanorods fabricated by a combination

of chemical vapor deposition and laser ablation were alsostudied.[199] It was found that some impurities (such as Er orMn) can result in preferential orientation of the nanorodsperpendicular to the substrate.[199] Doping is expected toinduce some changes in the morphology of the nanostruc-tures, although the effects have not been systematically in-vestigated for the majority of dopants and growth methods.In addition to intentional doping, fabrication of hierarchicalZnO structures where other elements are present in thesource material may result in the presence of secondaryphases and the incorporation of impurities in ZnO, as in thecase of Bi2O3-containing ZnO hierarchical nanostruc-tures.[192]

Group III and group IV elements are typically used todope ZnO with donor impurities. Halogen atoms can alsoserve as donors in ZnO, and have the additional benefit ofreducing oxygen adsorption on surfaces,[206] but they are lesscommonly used than group III dopants. Al-doped ZnO(AZO) films are commonly proposed as a transparent con-ductive oxide, which can replace for indium tin oxide elec-trodes in organic optoelectronic devices. AZO nanosheetsand nanowalls have been reported.[201] It was found that theaddition of Al2O3 to the source material resulted in Aldoping, but the morphology changed from nanowires tonanosheets and nanowalls, while low-temperature cathodo-luminescence spectra exhibited narrow donor bound-excitonlines, with the I6/8 line attributed to the Al donor,[201] inagreement with other reported assignments of this donorbound-exciton line.[76] Single-crystalline AlZnO nanowires/nanotubes were also reported, and it was found that incor-poration of Al resulted in an increased bandgap from3.29 eV to 3.34 eV.[194]

For the case of In doping, nanocrystalline films,[225] nano-rods,[198] nanobelts,[222] and nanowires[210,216] have been re-ported. ZnO and ZnO:In nanorods were fabricated fromthe reaction of zinc nitrate hydrate with hexamethylenetetr-amine (for In doping, indium chloride was added).[198] Nosignificant differences in the morphology of the nanorodswere observed after doping. A blue shift of the absorptionpeak and UV emission peak was observed, and the ratio ofUV-to-green emission decreased after doping.[198] On theother hand, broadening and a red shift of the UV emissionpeak was observed in In:ZnO nanobelts, which was attribut-ed to a significant increase in the carrier density.[222] A simi-lar red shift of the UV emission peak due to heavy Indoping was also observed in ZnO:In nanowires with a su-perlattice structure.[216]

In general doping with different donors produces broad-ening of the UV emission peak, but the peak shift is de-pendent on the dopant.[210] It was found that Sn doping pro-duces the largest red shift of the UV emission, as well as theappearance of strong green emission in doped ZnO nano-wires.[210] On the other hand, another study of optical prop-erties of Sn-doped ZnO nanowires reported no UV emissionshift (380 nm) and the appearance of new emission peaks at396, 461, and 502 nm.[219] Obviously, since both undoped anddoped ZnO can exhibit different optical properties depend-ent on the fabrication conditions, it is difficult to establishhow the properties will change after doping. The appear-




ance of new luminescence peaks is expected, but in the caseof significant increase in the carrier density, a red shift ofthe near band-edge emission is also expected. It was also re-ported that the optical properties of Sn-doped ZnO nano-belts were dependent on the type of flowing gas used, illus-trating the importance of the fabrication conditions.[208] Inaddition to Sn, Pb represents another possible donordopant,[219] and Pb-doped ZnO nanowires have been report-ed.[207] Pb doping was found to result in a red shift of theemission peak with the peak position dependent on Pb con-tent.[207] Sc doping also resulted in red-shifting of the PLspectra with increasing concentration, remarkably similar tothe results observed for Pb-doped ZnO nanowires.[213]

Acceptor dopants in ZnO are usually group V elements,such as N, As, and P. Nitrogen doping was reported in ZnOcrystals,[191] nanocrystals,[215] and nanorods.[204] N doping ofZnO crystals was found to result in the appearance of nitro-gen-associated donor–acceptor emissions in low-tempera-ture PL spectra.[191] While N-doped nanocrystals showedviolet luminescence at room temperature (attributed todefect emission),[215] N-doped nanorods showed a strong UVemission at 3.31 eV and negligible defect emission.[209] In thecase of As-doped ZnO nanowires, unusual behavior in tem-perature-dependent PL spectra has been observed, with ac-ceptor bound-exciton emission detectable at room tempera-ture.[203,214] On the other hand, p-type P-doped ZnO filmsexhibited more normal behavior with the acceptor bound-exciton peak dominant at low temperatures and free-excitonemission at room temperature.[190]

ZnO nanostructures have also been doped with differentrare-earth elements such as Tb,[228,229] Ce,[217] Eu,[200,204] andDy.[193] In the case of Tb-doped ZnO nanoparticles, emissionfrom both Tb and surface states was observed.[229] Tb emis-sion increased with increasing Tb concentration while theemission from surface states decreased.[228] Ce incorporationinto 1D ZnO nanostructures results in the appearance of aviolet-blue emission and the disappearance of any greendefect emission.[217] Eu-related emission was observed fromZnO:Eu nanorods for a suitable chosen excitation wave-length.[200,204] On the other hand, Dy-doped ZnO nanowiresexhibited only a UV emission of ZnO with only a very weakemission attributed to Dy.[193] Therefore, when doping withrare-earth elements, it is possible that their emission will bemasked by ZnO defect emission, and thus the excitationwavelength must be carefully chosen to establish the effectsof doping on optical properties.

Doping with transition metals is expected to result inchanges in the magnetic properties of the material, and Mn-doped ZnO has been predicted to be ferromagnetic at roomtemperature. Therefore, there has been considerable interestin the fabrication and characterization of transition-metal-doped ZnO nanostructures. Mn-doped nanocrystallinefilms,[227] tubes,[195] nanorods,[197] multileg nanostructures,[211]

nanobelts,[224] and tetrapods[223] have been reported. Otherdopants include Ni (nanowire arrays)[196] and Co (nanoclus-ter films).[202] Mn-doped ZnO rods were found to be ferro-magnetic at room temperature,[197] but both the magneticand optical properties of the Mn-doped nanostructures arestrongly dependent on the fabrication conditions. Mn

doping was found to quench green emission,[227] althoughother studies reported reduction in both UV and defectemission.[195] Decrease in UV emission and the appearanceof green emission after Mn incorporation have also been re-ported.[197] In addition, a blue shift and increase in intensityof the UV peak were found after Mn doping.[211] Very simi-lar spectra of ZnO and Mn-implanted ZnO were observedafter annealing an implanted sample at 800 8C.[224] A similarUV-to-green emission ratio has been observed in undopedand Mn-doped ZnO tetrapods.[223] Obviously, the change inthe optical properties is strongly dependent on the methodof incorporation of Mn, fabrication conditions, and proper-ties of undoped ZnO fabricated under similar conditions. Inthe case of Ni doping, a red shift of the UV emission withno significant change in the visible part of the spectrum wasobserved.[196] Co doping also resulted in a small red shift ofthe UV peak, as well as peak broadening.[202]

Other dopants whose effects on the optical properties ofZnO nanostructures have been studied includesulfur[205,212,221,226] and copper.[218, 220] Enhancement of greenemission with S doping has been reported,[205,221] as well asthe change in shape of the broad green defect emission.[212]

Either no significant shift[221] or, in contrast, a blue shift ofthe UV emission peak[205,212,226] has been reported. In case ofCu doping, broad PL spectra extending from the UV to thered spectral region were observed in Cu:ZnO nanowires.[220]

However, Cu-doped ZnO nanowires prepared by a differentmethod only showed an increased red shift of the emissionpeak with increasing copper content.[218] Therefore, it can beconcluded that regardless of the type of dopant, the opticalproperties of the nanostructures have a very strong depend-ence on fabrication conditions. Thus, very careful interpreta-tion of the measured spectra of doped ZnO nanostructuresis necessary.

6. Conclusions and Outlook

A great variety of ZnO nanostructures have been re-ported. Their optical properties have been studied mostly atroom temperature, although variable-temperature photolu-minescence studies on different ZnO nanostructures havenow been reported. Similar to other forms of ZnO, theorigin of different defect luminescence peaks remains unre-solved. In general, the optical properties of various nano-structures are very similar to those reported for thin films,and the observed size effects in nanorods and nanowires arethe result of different surface-to-volume ratios rather thanthrough quantum confinement. All of the defect emissionsreported in nanostructures can also be found in thin-film orbulk ZnO samples. The least-controversial defect emissionis the broad yellow luminescence commonly observed insamples prepared from aqueous solutions, which likely origi-nates from transitions involving interstitial oxygen; theorigin of other defect emissions require further study.

Although several methods for reducing or eliminatingdefect emissions, such as annealing in an Ar/H2 mixture foryellow emission and surface functionalization and hydrogentreatments for green emission, comprehensive study on the




relationship between fabrication conditions and visibleemission spectra is still needed. In order to conclusivelyidentify the nature of defects, it will likely be necessary tocombine PL studies with other experimental techniquessuch as EPR, carrier-concentration determination, and X-ray photoelectron spectroscopy (XPS). In particular, anneal-ing studies in different atmospheres and at different temper-atures may be a useful tool in establishing the origin of thedefect emission since annealing may result in the change ofoptical properties without changing the morphology.

As for the stimulated emission, while ultrafast carrierdynamics has been well studied in the EHP regime, furtherstudies are needed for lasing due to exciton–exciton scatter-ing, which is of higher practical interest. Also, for practicalapplications of ZnO lasers it is necessary to achieve electri-cally pumped lasing in ZnO. Closely related to this issue isthe achievement of reliable p-type doping with high carrierconcentrations and mobilities, which is difficult due to thepresence of native defects in ZnO. Although several groupshave reported p-type doping in ZnO, this issue is still underscrutiny.[1] Research on doped ZnO nanostructures has beenscarce compared to research on the doping of thin films andon undoped nanostructures. Finally, while there are numer-ous reports on the linear optical properties of ZnO nano-structures, much work remains to be done on studying theirnonlinear properties.

Acknowledgements

This work is partly supported by the Research Grant Councilof the Hong Kong Special Administrative Region, China (HKU7019/04P) and a University Development Fund grant of theUniversity of Hong Kong.

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Received: March 16, 2006Published online on July 18, 2006




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