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Guided Growth of Horizontal ZnSe Nanowires and their Integration into High-Performance Blue–UV Photodetectors

Eitan Oksenberg , Ronit Popovitz-Biro , Katya Rechav , and Ernesto Joselevich *

E. Oksenberg, Prof. E. Joselevich Department of Materials and Interfaces Weizmann Institute of Science Rehovot 7610001 , Israel E-mail: [email protected] Dr. R. Popovitz-Biro, K. Rechav Chemical Research Support Weizmann Institute of Science Rehovot 7610001 , Israel

DOI: 10.1002/adma.201500736

extended to new and interesting materials with a wider range of electrical, structural, and optical properties. Zinc selenide (ZnSe) is an important direct bandgap semiconductor with a wide 2.7 eV bandgap and a large 20 meV exciton binding energy. [ 26,27 ] It is interesting from the crystal structure stand-point due to reports of both zinc-blend (ZB) and wurtzite (WZ) NWs, and as an optoelectronic material, since it has a bandgap energy in the VIS range. [ 28,29 ] This chalcogenide holds promise for the fabrication of blue LEDs and lasers, and for diverse applications such as optical switching, photodetection, and second-harmonic generation (SHG) applications. [ 30–33 ]

Here we report the guided growth of horizontal and aligned ZnSe NWs with controlled orientations on six different fl at and faceted planes of sapphire (α-Al 2 O 3 ). The concepts and realiza-tion of the guide growth are presented in Figure 1 B. Three gen-eral growth modes that dictate the reproducible alignment of the NWs were realized: a) epitaxial guided growth along specifi c lattice directions on fl at surfaces, and graphoepitaxial guided growth on nanofaceted surfaces along: b) nanosteps, and c) nanogrooves.

The guided ZnSe NWs were fi rst examined with respect to their elemental composition, crystallographic orientation and growth directions. Surprisingly, some new behaviors of guided NWs were found: i) temperature-dependent directional growth of epitaxial NWs was encountered, where some growth direc-tions were observed only at elevated temperatures while others were observed throughout the entire guided growth tempera-ture regime; ii) crystal structure variations were observed where guided NWs on some sapphire planes presented exclusively one crystal phase, either WZ or ZB, while on other planes both ZB and WZ NWs could grow side by side.

In order to examine the optoelectronic properties of the ZnSe guided NWs, we exploited the advantages of the guided growth approach and fabricated multiple photodetectors using only parallel fabrication steps. The ZnSe-based photodetec-tors have the lowest dark currents (below our detection limit 10 −15 A) and the best measured rise and decay times (74 ms and 0.2 s, respectively) for devices that are based on ZnSe 1D nano-structures (see Table S1, Supporting Information, for previous results). [ 31,32,34–36 ] These results along with the simple and par-allel fabrication process and the ability to control the number of NWs in a single device and thus adjust its performance, reveal the advantages of guided ZnSe NWs as building blocks for fast blue and UV-light-sensitive photodetectors.

The vapor–liquid–solid (VLS) growth of guided ZnSe NWs was carried out in a two-zone tube furnace system (see the Experimental Section for details). NWs grow horizontally and aligned from the Au catalyst pattern edges or from dispersed nanoparticles. Their typical diameter varied between 10 and

Single-crystal semiconducting nanowires (NWs) have been the focal point of extensive and ongoing research, exploring both new physical phenomena and the compatibility of NWs as building blocks for various applications in electronics, [ 1 ] opto-electronics, [ 2,3 ] sensing, [ 4 ] and energy harvesting. [ 5,6 ] Amongst these devices, most of the planar ones were fabricated by a long, multistep and often serial process where each device was custom fabricated on a randomly located NW. In order to take the next step and enable the transition from single devices to large-scale functional systems, a deterministic con-trol over the location and orientation of multiple NWs is greatly needed. Several post-growth methods try to address this need with increasing success. [ 7–11 ] Nonetheless, each method has its drawbacks, which often include length limitations and insuffi cient control over the exact location of both ends of the NWs. The guided growth approach can offer an advantageous alternative that eliminates the need for post-growth align-ment methods. [ 12,13 ] It uses epitaxial and graphoepitaxial rela-tions with the substrate to guide horizontal NWs during the synthesis. This brings about well-aligned NWs that grow in specifi c and reproducible directions and crystallographic ori-entations as illustrated in Figure 1 A. Furthermore, combining top-down lithography processes with the unique features of guided growth, can realize the fabrication of multiple NW-based devices in a parallel manner. [ 14 ]

Growing interest in this approach is evident through increasing reports on successful horizontal growth of aligned NWs. However, thus far, the reported horizontally grown NWs provide a limited array of electrical and optical properties. In particular, with respect to their optical properties, ZnO, [ 12,15,16 ] GaN, [ 13 ] SnO 2 , [ 17 ] In 2 O 3 , [ 18 ] Sn-doped In 2 O 3 (ITO), [ 19 ] Mg 2 SiO 4 , [ 20 ] and TiO 2 [ 21 ] all have bandgap energies above the visible (VIS) range while the bandgap energies of Si, [ 22 ] GaAs NWs, [ 23 ] InAs, [ 24 ] and VO 2 [ 25 ] are below it. [22,23] Therefore, the bandgap energy repertoire of horizontally aligned NWs so far lacks the pivotal VIS range.

In order to unleash the potential of the guided growth approach and to explore its principles and generality, it must be

Adv. Mater. 2015, DOI: 10.1002/adma.201500736

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100 nm, and their length could reach up to 100 µm. Thin (50–100 nm) slices were cut across the NWs using a focused ion beam (FIB) and later examined under a high-resolution transmission electron microscope (HRTEM) in order to explore their elemental composition, crystallographic structure, and epitaxial relations with the substrate.

The guided NWs were found to be composed of Se and Zn with an atomic ratio of 1.13:1 (Figure S1, Supporting Infor-mation). The presence of Zn vacancies, which is indicated by the atomic ratio, is not uncommon for ZnSe NWs. Spe-cifi cally, using a Zn rich atmosphere during the growth or for post growth annealing was reported as successful routs to avoid these vacancies when needed. [ 37 ] The crystal phase as well as the morphology and orientation of the guided NWs varied with the different orientations of the fl at sapphire sur-faces as depicted in Figure 2 A–E, and detailed in the next paragraphs.

On C (0001) sapphire the guided ZnSe NWs grow along six isomorphic M ⟨1010⟩ directions that refl ect the threefold

symmetry and three glide planes of the substrate. The NWs grown on C-plane have a ZB crystal structure and their growth axis is along the [110] direction (Figure 2 A). Strain due to the mismatch between the NWs and the surface is relieved by misfi t dislocations. These dislocations were observed in a lattice ratio that is compatible with the minimized mismatch along its transversal cross section, as was seen previously with ZnO and GaN guided NWs. [ 12,13 ] In order to further verify the crystal-lographic orientation of the NWs and to evaluate their crystal quality, a single NW was sectioned along its growth axis. Only few plane defects were observed along the full length of the NW, less than one stacking fault per micrometer, revealing a virtually perfect single-crystal structure (Figure S2, Supporting Information).

When exploring the guided growth of ZnSe NWs on R (1102) sapphire, we found that the NWs grow along four different isoperiodic directions of the sapphire substrate, the ±[2201] directions and in 94° to the ±[0221] directions. Contrary to our above-described results on C-plane, here the guided NWs



Figure 1. Guided growth of horizontal ZnSe NWs: A) Concept and realization – schematic illustration of guided VLS growth (lower) versus conven-tional VLS growth (upper). B) Three modes of guided growth (schematic cross-section views and cross-sectional HRTEM images of their experimental realization): a) epitaxial growth along specifi c lattice directions, b) graphoepitaxial growth along L-shaped nanosteps of an annealed miscut substrate, and c) graphoepitaxial growth along V-shaped nanogrooves of an annealed unstable low-index substrate. C,D) SEM images of guided ZnSe nanowire arrays grown on R (1102) sapphire. E) Guided ZnSe NWs grown on annealed M (10 10) sapphire. The light-blue indices and vectors describe the sap-phire crystallographic directions.

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exhibit exclusively a WZ crystal structure with a dominant [0001] growth axis orientation (Figure 2 B). This reproducible and aligned growth of ZnSe NWs with a polar orientation can be used for relevant polarity-dependent applications, including non-linear optics, where the ZnSe strong SHG response can be harnessed. [ 38 ] Interestingly, the observed growth directions of guided ZnSe NWs on R-plane are different than the reported growth directions of carbon nanotubes and guided GaN and ZnO NWs on this plane. [ 12,13,39 ] These results demonstrate that the direction of guided growth is not only substrate dependent

but depends also on the nature of the material and its relation-ship with the substrate.

Surprisingly, the guided growth of ZnSe NWs on A (1120) sapphire revealed a new phenomenon: For the fi rst time, a temperature dependent directional growth of guided NWs is reported. For a 700 °C growth temperature, the NWs grow along two dominant directions that correspond to the ±[4401] directions of the sapphire substrate (Figure 2 C). However, when the samples are placed at a higher temperature, the NWs grow along four dominant directions: the ±[4401] directions that are



Figure 2. A–E) Epitaxial guided growth of ZnSe NWs on fl at sapphire substrate. For each substrate orientation (the light-blue indices, vectors and bars describe the sapphire crystallographic orientations whereas the yellow ones describe the ZnSe crystallographic orientations): (a) A schematic model of the directional growth. (b) SEM images of the experimental realization and growth directions. (c) Cross-sectional HRTEM images and the crystal-lographic orientations of the guided NWs and the substrate. (d) Magnifi cation of the NW-surface interface and the selected area fast Fourier transform of the ZnSe section in the HRTEM image.


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seen at lower temperatures and the ±[2201] directions at a 120° angle. Interestingly, although these growth directions have dif-ferent lattice parameters, all the NWs exposed the same (1101) horizontal plane to the substrate, and revealed a WZ crystal structure with the same [1102] growth axis (Figure 2 D). This temperature-dependent growth is reminiscent of a previously reported similar phenomenon for vertical NWs. [ 40 ] If this phenomenon proves to be general, one could use the growth temperature to choose certain growth directions of the NWs that are desirable for a specifi c application. Alternatively, the temperature may be changed during the growth process to infl ict a kinked or a zig-zag morphology on the NWs.

The growth of guided ZnSe NWs on non-annealed M (1010) sapphire had some surprising similarities to the growth on A-plane. The NWs grow along four dominant directions, the ±[0001] growth directions (with a 5° angle toward ±[ 1120 ]) and the ±[1212] directions at a 130° angle (Figure 2 E). Similar to the behavior on A-plane at high temperatures, the growth directions are not isoperiodic. Yet, again all the NWs exposed the same (1101) horizontal plane to the sapphire substrate, and revealed only a WZ crystal structure with the same growth axis [1102]. The emergence of identical crystallographic orientation

for guided ZnSe NWs on two different sapphire planes might suggest a relatively dominant energetic advantage to this crys-tallographic orientation and morphology. Perhaps the energetic gain for this morphology, exposing these specifi c planes to the substrate and environment, is larger than the energetic gain from minimizing the epitaxial mismatch. With that said, the substrate still determines exactly four growth directions, hence some of the energetic considerations of the NW-substrate inter-face remain signifi cant.

In efforts to gain even greater control over the growth of the NWs, graphoepitaxial growth was also explored. Past studies have shown that NWs grow preferentially along nanosteps and nanogrooves on nanofaceted surfaces resulting from the self-reconstruction of unstable planes upon annealing at high tem-peratures. [ 41 ] Moreover, the graphoepitaxial effect proved to be dominant over the epitaxial effect. [ 12,13 ] A similar behavior was seen here for ZnSe guided NWs, as depicted in Figure 3 . The six-directional growth of ZnSe NWs on fl at C-plane can be over-ruled with the creation of nanosteps. A 2° miscut toward [1100] followed by annealing produces an array of L-shaped nanosteps due to step-bunching of the high-index planes that are exposed. The NWs are guided by these steps and grow along them in the



Figure 3. Graphoepitaxial effect on the horizontal growth of ZnSe NWs on: A) C (0001) sapphire (well-cut vs miscut) and B) M (10 10) sapphire (non-annealed vs annealed). For each plane (the light-blue indices, vectors, and bars describe the sapphire crystallographic orientations whereas the yellow ones describe the ZnSe crystallographic orientations): a) SEM images of the epitaxial guided growth on the fl at corresponding surface. b) Schematic illustration of the graphoepitaxial effect. c) SEM image of bidirectional growth on the faceted surface and d) a higher magnifi cation image that reveals the nanofaceted surface. e) Cross-sectional HRTEM images and the crystallographic orientations of the guided NWs and the substrate and in the inset, the selected area Fourier transform of the NW. f) High magnifi cation image of the NW-substrate interface.


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±[1210] directions (Figure 3 A). Unlike the guided ZnSe NWs grown on fl at C-plane. These NWs do not have a smooth and uniform surface, and their crystallographic analysis revealed either ZB or WZ crystal structure with a variety of crystallo-graphic orientations (Figure S3, Supporting Information).

Flat M (1010) sapphire is thermodynamically unstable, and undergoes restructuring at elevated temperatures to present the more stable S-plane and R-plane in well-structured periodic nanogrooves. On the faceted M-plane surface, the horizontal ZnSe NWs grow aligned following these nanogrooves in the ±[1210] directions of the sapphire substrate (Figure 3 B). Crys-tallographic analysis revealed a tendency toward a ZB crystal phase with a [110] growth axis. Still, about 40% of the exam-ined NWs had different growth axes and sometimes NWs with WZ crystal phase existed side by side with the more frequent ZB [110] orientation. The relative variance in graphoepitaxial growth compared with fl at surfaces, which was observed also with GaN and ZnO NWs, [ 12,13 ] may be attributed to the need to adapt simultaneously to the adjacent R-plane and S-plane fl anks of each nanogroove and their different energetic constraints. This hypothesis is supported by growth results on multiple smaller nanogrooves (Figure S3, Supporting Information).

The guided growth horizontal ZnSe NWs on all these dif-ferent substrates demonstrates the generality of the guided growth phenomenon and provide rich empirical data toward its understanding. Although the exact mechanism of guided growth is not yet fully elucidated, a few recent models have rationalized the horizontal growth of NWs, either by consid-ering a fully faceted limit of a continuum model taking into consideration facet dynamics, droplet statics, and the energy required to introduce a new facet, [ 42 ] or by introducing thermal fl uctuations to a thermodynamic description. [ 43 ]

The guided growth approach utilizes surface–NW interac-tions to guide horizontal NWs in specifi c orientations. Ini-tially, some concerns regarding possible degradation of their physical properties were raised due to strain-related defects induced by these surface–NW interactions. Later, these con-cerns were refuted by previous studies in our group for ZnO and GaN guided NWs. [ 12,13 ] In order to further test whether the physical properties of guided NWs are not inferior relative to unbounded ones, and to explore the optoelectronic proper-ties of ZnSe guided NWs, multiple photodetectors were fab-ricated. Exploiting our deterministic control over the location and the growth direction of the guided NWs, the photodetec-tors were fabricated using only parallel fabrication steps. Ti/Au (10/50 nm) electrodes were deposited following the guided growth process as seen in Figure 4 A,B. A typical device fab-ricated over guided NWs on annealed M-plane has ≈30 par-allel NWs with a typical width of 30 nm that cross a 2–10 µm channel. For ZnSe with a 2.7 eV (460 nm) bandgap, we used a 405 nm laser to illuminate above this energetic threshold. The I–V characteristics of a typical device illuminated with different intensities and under dark conditions are presented in Figure 4 C. The photodetector is stable throughout a wide range of source–drain voltages and the current increases almost linearly as the voltage increases indicating nearly Ohmic contacts. Under 13 mW cm −2 illumination and 30 V bias, a 10.7 pA current was measured. The resistivity, which is on the order of ≈10 5 Ω cm, is consistent with the range of bulk

ZnSe values [ 44,45 ] and ZnSe nanostructure values as displayed in Table S1, Supporting Information. Our 10 fA detection limit did not enable a full quantifi cation of the extremely low dark current of the device, thus we can only assess as a lower limit that the on/off ratio is at least three orders of magnitude.

In order to characterize the time-dependent response, the devices were exposed to 20 s cycles of light-off, light-on conditions under 13 mW cm −2 illumination and 30 V bias. Figure 4 C depicts a typical current response to these conditions. The almost identical response throughout 10 on/off cycles indicates good device stability and complete reversibility of its on and off states. Rise and decay time of 0.07 and 0.2 s, respectively, were calculated by fi tting with an exponential



Figure 4. Optoelectronic behavior of the guided ZnSe NWs: A) A sche-matic illustration of the device fabrication process. Ti/Au electrodes are deposited over guided ZnSe NWs that grow from a patterned Au catalyst. B) SEM images of the fabrication steps illustrated in (A). C) I–V charac-teristics of a typical device under different 405 nm illumination intensities and under dark conditions. D) The response of a typical photodetector set at a 30 V bias to on/off switching of a 405 nm laser illumination (13 mW cm −2 ).


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function. To the best of our knowledge, these are the best response times of all nanostructure-based ZnSe photo detector reported so far (see Table S1, Supporting Information). Further-more, these values are amongst the best among other photo-detectors that are based on 1D nanostructures of different materials, including ZnO, Cu 2 O, and CdTe. [ 46,47 ] Reports of higher dark currents, and degraded stability of photodetectors based on periodically twinned ZnSe NWs suggest that the ultralow dark current and fast response times may be attributed, at least in part, to the high quality of the ZnSe single crystal. [ 24 ]

The performance of our optoelectronic devices indicates that the physical properties of these guided ZnSe NWs do not suffer from degradation due to interaction with the substrate. In other respects, producing well-aligned NWs that grow in specifi c and reproducible directions and crystallographic orientations enables parallel fabrication of multiple devices in a confi gura-tion of choice. Moreover, the performance of each device can be adjusted with relative ease by changing the size of the cata-lyst pattern and the growth parameters, and thus controlling the number of NWs in each device. [ 14 ] The ability to simulta-neously fabricate a large number of devices base on one, sev-eral or many NWs with the same crystallographic orientation represents a high degree of control and uniformity, which are necessary both for research and applications. Even though the performance of these photodetectors is still not comparable with that of commercial high-end devices, we believe that these initial results prove the advantages of guided growth for the large-scale integration of NWs into optoelectronic devices.

In summary, we have demonstrated the guided growth of horizontal ZnSe nanowires with controlled orientation on six different fl at and faceted planes of sapphire (Table S3, Sup-porting Information). Both epitaxial growth along specifi c lat-tice directions on fl at surfaces and graphoepitaxial growth along nanosteps and nanogrooves were realized. We found some new guided growth phenomena including temperature-dependent directional growth and the dependence of crystal phase, either WZ or ZB, on the substrate and growth conditions. Next, we exploited the deterministic control over the growth direction and the crystallographic orientation of the guided ZnSe NWs to fabricate multiple photodetectors using a parallel and simple process. Their performance, including the lowest dark currents and the shortest response times for any nanostructure-based ZnSe photodetector, indicates that the optoelectronic proper-ties of horizontally grown ZnSe NWs are not degraded by the interaction with the substrate. Furthermore, these values are impressive compared to other photodetectors based on similar 1D nanostructures. The simultaneous fabrication of a large number of devices base on one, several or many NWs in the same crystallographic orientation, polar, nonpolar or semipolar, along with the demonstrated physical properties, indicates that guided NWs are promising building blocks for versatile high-performance devices that are easy to fabricate and scale up. All these benefi ts can be well harnessed both for research and applications. Finally, the introduction of the fi rst chalcogenide guided NWs could lead the way to other chalcogenides with their diverse electronic and optoelectronic properties. They will surely increase the diversity of the composition, structure and properties available with guided NWs. Preliminary results indi-cate that we can use grow guided NWs of other chalcogenides

such as ZnTe, CdSe, and CdS (unpublished results). The growing list of materials and proven generality of the guided growth approach, establishes it as a promising pathway toward the large-scale integration of NWs into functional systems.

Experimental Section Nanowire Synthesis : The substrate preparation and catalyst deposition

and patterning are describe previously [ 12 ] and specifi ed in the Supporting Information. The VLS growth was carried out in a two-zone tube furnace system. The system was purged with a N 2 (99.999%, Gordon Gas) and H 2 (99.99995%, Parker Dominic Hunter H 2 generator) 49:1 mixture and maintained at a fl ow of 500 sccm and at 400 mbar. The Zn and Se elements originated from a ZnSe powder (99.99% SPI Supplies). During the growth process, the ZnSe source powder was held at 970 °C at the fi rst heating zone, while the samples were placed downstream in the second heating zone and held at 700–750 °C. Growth time was typically 15–40 min. NWs and nanobelts grow vertically in the center of the Au catalyst pattern while horizontal and aligned NWs grow from the pattern edges where the clean sapphire surface is accessible. The vertical nanostructures were removed by a mild sonication in isopropyl alcohol for a few seconds. Dispersed Au nanoparticles (10–50 nm) can also be used to achieve a similar growth of guided NWs with better control over the catalyst nanoparticle size. When using a diluted solution of Au nanoparticles, the clean sapphire surface is accessible to each nucleation site. Under these conditions, the horizontal growth is dominant over the vertical growth.

Microscopy Characterization : The NWs were imaged using fi eld-emission SEM (Supra 55VP FEG LEO Zeiss). The thin (50–100 nm) lamellae for HRTEM characterization were made using a FEI Helios 600 Dual Beam microscope and inspected using a FEI Tecnai F30-UT fi eld-emission TEM, equipped with a parallel EELS (Gatan imaging fi lter) operating at 300 kV. TEM digital images were recorded using a Gatan Ultrascan1000 CCD camera. The images were analyzed to determine crystallographic orientation and epitaxial relations using fast Fourier transform (FFT) from selected areas in the nanowire cross sections. Indexing of the FFT peaks was done according to crystallographic tables for bulk ZnSe.

Device Fabrication and Characterization : A Photolithography mask was designed to defi ne an electrode pattern that is compatible to the catalyst pattern of the guided NWs. Ti/Au (10/50 nm) electrodes were deposited over the guided NWs using electron beam deposition.

The optoelectronic characterization was done in air and RT using a Janis ST-500 probe system with a Keithley 4200-SCS. A 405 nm laser diode module (Laser Components) was used to illuminate the device and the light intensity was calibrated with a LM-2 VIS silicon photodiode optical sensor (Coherent). For the on/off measurements, a square wave function was transmitted using a function generator to a beam blanker with response times of <0.3 ms. A 30 V bias was used in order to obtain a high signal-to-noise ratio suitable for accurate time-dependent analysis.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors thank Dr. D. Tsivion and N. Shadmi for helpful discussions. This research was supported by a European Research Council (ERC) Advanced Grant No. 338849, the Israel Science Foundation, Minerva Stiftung, Kimmel Center for Nanoscale Science, Moskowitz Center for


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Nano and Bio-Nano Imaging, and Perlman Family Foundation. E. J. is incumbent of the Drake Family Professorial Chair in Nanotechnology.

Received: February 11, 2015 Revised: April 21, 2015

Published online:

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Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2015.

Supporting Information

for Adv. Mater., DOI: 10.1002/adma.201500736

Guided Growth of Horizontal ZnSe Nanowires and theirIntegration into High-Performance Blue–UV Photodetectors

Eitan Oksenberg, Ronit Popovitz-Biro, Katya Rechav, andErnesto Joselevich*

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DOI: 10.1002/adma.201500736 Supporting Information Guided Growth of Horizontal ZnSe Nanowires and Their Integration into High-Performance Blue-UV Photodetectors Eitan Oksenberg, Ronit Popovitz-Biro, Katya Rechav and Ernesto Joselevich* E. Oksenberg, Prof. E. Joselevich Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 7610001, Israel E-mail: [email protected] Dr. R. Popovitz-Biro, K. Rechav Chemical Research Support, Weizmann Institute of Science, Rehovot 7610001, Israel Supporting Information

Table S1. Photodetectors based on1D ZnSe nanostructurs.

Photodetector Nanostructure Illumination light (nm)

Bias (V)

Dark current or

conductance

Photocurrent or

conductance

Rise time

Decay Time

Ref.

ZnSe Nanoribbon 400 30 <10-4 pA 1.89 pA <0.3s <0.3 s 31 ZnSe nanowire 400 8 ≈180 pA – 32

ZnSe/SiO2 Core/.Shell nanowire

514.5 2 70.5pS 123 pS <8 s <8 s 34

ZnSe nanowire 325 0.1 29nA 2.7 µA <1 s <1 s 35 ZnSe:Sb nanowire 365 -5 4.3nA ≈0.26 µA <1 s <1 s 36

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Figure S1. Compositional analysis. (a) TEM energy-dispersive X-ray spectroscopy of a typical guided ZnSe nanowire on annealed M ( 0110 ) sapphire and the examined nanowire in the inset (b) Scanning TEM energy-dispersive X-ray spectroscopy of a typical nanowire on A ( 2011 ) sapphire. The examined nanowire and the scanning pathis in the inset.

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Figure S2. Structural analysis of a ZnSe guided NW on C (0001) sapphire. Cross sections (A) along and (B) across a guided NW on C-plane sapphire: (a) a model of the cross section (b) HRTEM image of the cross section. (c,d) Enlarged images of the interface between the NW and the substrate, in the inset of the transversal lower image, a Fourier filtered version of the interface is presented, highlighting the periodic misfit dislocations in a 6:5 ratio between ZnSe and sapphire lattices. A structural model is fitted to the HRTEM images.

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Figure S3. The effect of the faceted surface on the morphology and single crystal quality of ZnSe NWs guided by (A) a single nanogroove on annealed M-plane (B) multiple nanogrooves on annealed M-plane and (C) nanosteps on annealed miscut C-plane. For each surface: (a) SEM image showing the morphology of the NW. (b) Cross sectional HRTEM images and (c) higher magnification HRTEM images and the corresponding selected area Fourier transform of the ZnSe NWs. Further discussion on the effect of the faceted surface on the morphology and single

crystal quality of the guided ZnSe NWs.

The graphoepitaxial guided ZnSe NWs have variance in the growth axes and a rougher

surface compared with epitaxial guided growth on flat surfaces. This may be attributed to the

need of the NWs to adapt simultaneously to multiple planes and their different energetic

constraints. This hypothesis is supported by comparing the growth results on annealed M-

plane when the NWs are guided by a single nanogroove (Figure S3A) and when the NWs are

guided by multiple smaller nanogrooves (Figure S3B). In the former case the NWs are

relatively smooth and have a dominant ZB crystal phase and a dominant [110] growth axis. In

the latter case the NWs have to accommodate simultaneously to multiple nanogrooves and

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multiple R and S-planes. These highly strained NWs have a rough and distorted morphology,

they develop crystallographic defects and show no uniformity in their growth axes and crystal

phase. In the case of grapoepitaxial growth on miscut C-plane the NWs are usually guided by

multiple nanosteps. These NWs are also relatively rough with different growth axes and

crystal phases. To conclude, it seems that when more planes participate in the guidance of the

NWs, more energetic constraints must be accounted for and more stress is accumulated in the

NW. The strain is manifested in a rougher surface and lesser uniformity in growth axes.

Table S3. Epitaxial Relations and Lattice Mismatch for Guided ZnSe Nanowires on Different Sapphire Surfaces

Substrate orientation

#of NWs (total #

examined)

Crystal phase

Horizontal planes ZnSe || sapphire

Axial direction ZnSe || sapphire

Minimal Mismatch

transversal

Minimal Mismatch

axial

C ( 0001) 4(4) ZB ( 001) || ( 0001) [110 ] || [ 0101 ] 5.17% 8.12%

R ( 0211 ) 15(18) WZ ( 0110 ) || ( 0211 ) [ 0001] || [ 0122 ] 1.70% 4.84%

A ( 0211 ) 18(20) WZ ( 0211 ) || ( 0211 ) [ 0211 ] || [ 1044 ] 15.15% 12.24%

A ( 0211 ) high temp

5(7) WZ ( 0211 ) || ( 0211 ) [ 0211 ] || [ 0122 ] 8.53% 7.60%

M ( 0110 ) 4(4) WZ ( 0211 ) || ( 0110 ) [ 0211 ] || [ 2121 ] 4.24% 5.97%

M ( 0110 ) 6(6) WZ ( 0211 ) || ( 0110 ) [ 0211 ] || [ 0001] 2.82% 5.97%

Annealed M 7(12) ZB [110 ] || [ 1021 ] Miscut C towards [ 0011 ]

6 ZB or WZ

[XXXX] || [ 1021 ]

Experimental Section Substrate Preparation: Singular, well-cut, sapphire wafers were purchased from Roditi

International. Miscut C-plane (0001) sapphire wafers were custom-produced by Gavish Ltd.

Miscut C-plane and M-plane were annealed at 1400 and 1600 °C, respectively, in air, using a

high-temperature furnace (Nabertherm). Prior to use, the substrates were cleaned as described

previously.[12]

Catalyst Deposition and Patterning: The Au catalyst was usually deposited by electron-beam

evaporation of a thin (5 Å) metal layer. First, the catalyst pattern was defined using standard

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photolithography, next a thin metal film was deposited followed by a lift off process. Heating

to 550 °C in air generated nanoparticles that served as catalyst for the VLS growth. In

addition, dispersing commercially available Au colloidal suspensions (British Biocell

International) was used, obtaining similar results.

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