The key role of surface in semiconductor nanostructures for photovoltaics and sensing applications

Dr. Mirabella SalvatoreConsiglio Nazionale delle Ricerche - Istituto per la

Microelettronica e Microsistemi,Università di Catania,

Dip. Fisica e Astronomia,Catania, Italy

Hosted by Prof. Barmak

The key role of surface in semiconductor nanostructures

for photovoltaics and sensing applicationsIn the current nanotechnology age a countless amount of “small and smart”solutions are investigated and proposed in real devices for a variety of applications,such as photovoltaics, sensing, catalysis, security, environment, biomedical area ...The exploitation of nanostructures (NS) usually benefits from enhanced surfaceover volume ratio and from quantum confined effects arising at the nanoscale. Still,surface states and interface defects quite often overwhelm the previous effectsand ask to be comprehended in details for a full utilization of NS. In this talk I’ll givefew examples of surface/interface effects in semiconductor NS (Si and Ge QDs, ZnOand NiO low-cost NS) applied to photovoltaics and sensing.

The light absorption in Si and Ge NS embedded in insulators will be presented,evidencing whether and to which extent the quantum confinement effectinfluences the light-matter interaction. An ideal size tuning of the optical bandgapis achieved only if Ge quantum dots have thin and defect-free Ge/SiO2 interface.Indeed, an unprecedented high absorption efficiency, ten times larger than in thebulk, is obtained for smaller and well-ordered Ge QDs in a superlattice approach.

Despite the huge potential in disposable sensors, low-cost NS of transition metaloxides (TMO) often lack reproducibility and stability because of the growthmethod. The growth mechanism of ZnO nanorods and nanowalls by means ofchemical bath deposition will be presented and modeled, as well as the surfacestates responsible for UV- and pH- sensing will be evidenced. Finally, a facilesynthesis on plastic substrate of a large surface area, NiO NS will be described andapplied as a high sensitivity, non enzymatic glucose sensor.

Friday, December 11, 201511:00 a.m.

Room 214 S. W. Mudd

SALVO MIRABELLA CATANIA, ITALY

THE KEY ROLE OF SURFACE IN SEMICONDUCTOR NANOSTRUCTURES FOR

PHOTOVOLTAICS AND SENSING APPLICATIONS

Catania

J. DANCKERTS, XVII sec.

04 Dec. 2015

www.matis.imm.cnr.it

Nanostructures

1434

http://www.matis.imm.cnr.it/

Columbia University, MSE colloquium, New York [email protected] 4/44

Laurie McCormik

Nanostructures

mailto:[email protected]


OUTLINE

0D

1D

2D

Ge QDs: not only size matters

ZnO NRs: surface defect engineering

ZnO NWs: flexible pH sensor NiO NF: flexible glucose sensor



OUTLINE

0D

1D

2D


ZnO NRS: surface defect engineering




Ge QDs for PV

Ge vs. Si • Higher absorption coefficient • Larger size range for QCE • Bandgap tuning within solar spectrum Si QDs

Ge QDs

Multi-junction SC All Si tandem solar cell “dream”



Light absorption in Ge QDs

S. Cosentino et al. APL (2011) P. Liu et al. JAP (2012) S. Mirabella et al. APL (2012) S. Cosentino et al. NRL (2013) S. Mirabella et al. APL (2013) S. Cosentino et al. JAP (2014) S. Cosentino et al. SOLMAT (2015) E. Barbagiovanni et al., JAP (2015) S. Cosentino et al., Nanoscale (2015)



Light absorption in Ge QDs

1 2 3 4 510-19

10-18

10-17

PECVD QDs (3.5 nm) PECVD QDs (4.4 nm) Sputter QDs (3 nm) Sputter QDs (4 nm)

Abso

rptio

n cr

oss

sect

ion

[cm

2 ]

Energy [eV]• Blue shift with decreasing QD size • Greater shift in PECVD w.r.t. sputter

Dtασ =

• Ge QDs (2-8 nm) embedded in SiO2 • Ge QDs density (~ 1018 cm-3) • Surface to surface (S2S) distance (1-3 nm)

absorption (α) absorption cross section (σ): photon absorption probability per Ge dose

SiGeO film

PECVD or sputter (deposition 250°C: 8 - 20% Ge)

(600-800°C annealing in N2)



Optical bandgap variation

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk

Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]


Opt

ical B

andg

ap [e

V]

• Eg vs size depends on synthesis technique … • Is there any role of interface ?

( ) ( )2optg

Tauc EB−⋅= ω

ωωα

Tauc law

VB

CB

VB

CB

Ge/SiO2 Ge/GeO2

V0,e 2.8 eV 1.2 eV

V0,h 4.5 eV 3.6 eV

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA

Opt

ical B

andg

ap [e

V]

( )2

0*2*

22

21

2)(

−

+⋅+=

VmrLmbulkEQDE gg

π

• Eg vs size depends on synthesis technique … • Is there any role of interface ? • Any difference in the interfaces ?

SiO2 SiO2 Ge QD

SiO2 SiO2 Ge QD

GeO2



2 nm

Ge QD

TEM analysis

Z contrast profiling reveals systematically thinner interfaces in PECVD samples 2

1)(1)( 0 QDdiameterexf

xx≥Γ

+=

−Γ

−−

3.5 nm QD



EELS-STEM analysis

0.0

0.2

0.4

0.6

0.8

1.0

5 10 15 20 25 30 35 40 45 50 55 600.0

0.2

0.4

0.6

0.8

1.0

Sputter

Inte

nsity

[a.u

.]

PECVD

Inte

nsity

[a.u

.]

Energy [eV]

EELS core QD Fit

interband transition Ge Ge QD vol. plasmon SiO2 vol. plasmon Ge-Ge M4,5 band Ge-O M4,5 band

AGe-O

AGe-Ge

AGe-pl )( plGeGeGe

OGeOGe AA

AF−−

−− +

=

FGe-O ~ 16 % for sputter FGe-O ~ 8 % for PECVD

STEM: e-beam probe a cylinder of ~ 40 Ge atoms, 3 of which at surfaces

• Significant Ge-O surface contribution • Greater Ge-O contribution in sputter samples • Thinner interface in PECVD samples

e-beam

• How to consider these interfaces features in the QCE of Eg variation ?



SPDEM model

E. G. Barbagiovanni, et al., J. Appl. Phys. (2012), 111, 034307 E. G. Barbagiovanni, et al. Physica E, (2014), 63, 14–20 E. G. Barbagiovanni, et al., J. Appl. Phys. (2015), 117, 154304

Confining potential breaks the translational symmetry new momentum operator (pγ) effective mass spatially dependent

m(x)~1/(1+γx)2 γ~1/D as D decreases m(x) decreases confinement energy increases



Interface effect on bandgap

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]


Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA

Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA SPDEM PECVD SPDEM Sputter

Opt

ical B

andg

ap [e

V]

Ge/SiO2 Ge/GeO2 PECVD Sputter

V0,e 2.8 eV 1.2 eV 1.1 eV 0.9 eV

V0,h 4.5 eV 3.6 eV 3.3 eV 2.8 eV

( )( )

+

⋅+= *

,

,*,

,

23

hc

hc

ec

ecbulkgg m

VmV

DDEDE

µ

SPDEM model well accounts for the different Eg variation in the two samples

S. Cosentino, et al., Nanoscale (2015), 7, 11401

QD

SPDEM model



Ge QDs - conclusion

0D Ge QDs: not only size matters

Interface states affect the quantum confinement PECVD samples closer to ideal QCE (Ge/GeO2)

E. G. Barbagiovanni, et al., J. Appl. Phys. (2015), 117, 154304 S. Cosentino, et al., Nanoscale (2015), 7, 11401

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA SPDEM PECVD SPDEM Sputter

Opt

ical B

andg

ap [e

V]



OUTLINE

0D

1D

2D






Global connections

Exponential growth of devices connected through IoT!



Sensors invasion

Need for low-cost, massive and controlled production of nanostructures for future sensing applications



OUTLINE

0D

1D

2D






Step One

Seed Layer

Zinc Acetate in ethanol

Spin coating + Heating: 240 °C, 20 min

Hot Plate

Step Two Growth

90°C, 1h Aqueous bath: [Zn(NO3)2 ⋅6H2O] 25 mM

[C6H12N4] 12.5-50mM

L.Vayssieres et al., J.Phys. Chem. B, 105, 3350 (2001) L.E. Greene et al., Angew. Chem.I nt.. Ed. 42, 3031 (2003)

Chemical bath deposition

Effect of [HMTA]: pH or chelating ?

300 ZnO seeds/µm2

Hot Plate

Magic formula



ZnO NRs: HMTA effect



ZnO NRs: HMTA effect

V. Strano, et al., J. Phys. Chem. C (2014) 118, 28189

HMTA effect: pH and steric hindrance



Laser irradiation of ZnO NRs Adrian M. Chitu, prof. J. Im’s group - CU G. Fiaschi, Y. Komen, Y. Shacham – TAU Laser energy flux: 100-1000 mJ/cm2

950 770 550 225 mJ/cm2



Laser irradiation of ZnO NRs

580 mJ/cm2

310 mJ/cm2

Progressive melting from the top



ZnO NRs and light

Below gap light

Above gap light

Light emission

Light scattering

V. Strano, et al., Appl. Phys. Lett. (2015)

E. G. Barbagiovanni, et al., Nanoscale (2015)



ZnO NRs: light emission

Near band edge emission: no CBMVBM transition, FX-D (Zni donor state)! Visible band: defect states in the band gap …



Defect engineering

ERDA H detection

PLE

Visible PL: four components (BGOR) fit, Orange line always present. PLE: excitation onset changes with annealing. ERDA: H trapped at surface, released at high T.



BGOR model

E. G. Barbagiovanni, et al., Nanoscale (2015)

Surface defect engineering allows modulation of visible PL in ZnO nanorods



ZnO NRs: UV sensing

E. G. Barbagiovanni, et al., Appl. Phys. Lett. (2015)

PL transient only for green line (surface O vacancy). PL transient = IV transient Space for optical sensing of gas



ZnO NRs - conclusion

1D ZnO NRS: surface defect engineering

Low-cost but controlled ZnO NR synthesis (HMTA role) Laser annealing induced modification Surface defect engineering for sensing

V. Strano, et al., J. Phys. Chem. C (2014) V. Strano, et al., Appl. Phys. Lett. (2015) E. G. Barbagiovanni, et al., Nanoscale (2015)

E. G. Barbagiovanni, et al., Appl. Phys. Lett. (2015)



OUTLINE

0D

1D

2D






ZnO nanowalls

Growth upon Al film

90°C, 1h Aqueous bath: [Zn(NO3)2 ⋅6H2O] 25 mM

[C6H12N4] 12.5-50mM

4 µm

TOP VIEWS

200 nm

1 µm

TILTED VIEW



ZnO NWs: growth A

l(OH

) 4-

Al(O

H) 4

-

Al(O

H) 4

-

Substrate Al

Substrate Al

Substrate Al

Growth Time

φ s

K. O. Iwu, et al., Cryst. Growth Des. (2015)



• Al/ZnO nanoporous provides a perfect conducting/selective layer for extended gate

• Al/ZnO bi-layer was connected to the gate of an LTPS TFT fabricated on flexible PI

pH sensor

Extended Gate Thin Film Transistor (polycrystalline silicon at low temperature)

ZnO NW



pH sensor

Basic test for further biosensors (as Enzyme FET) involving pH variation due to Enzyme-Analite reaction

- Measurements were performed @ 25 °C in dark condition and after 10 min after changing pH solution

- Reference electrode Ag/AgCl - IdVg performed @ Vds=0.1 V with slow ramp rate (2 s/V) - IdVds performed @ Vg=9 V

- pH-sensitivity nearly 60 mV/pH, close to ideal

Nernstian response (2.3 KT=60 mV/pH)) L. Maiolo, et al., Appl. Phys. Lett. (2014)



Future glucose sensor

Today Tomorrow

Less invasive Stable Low-cost Flexible Non toxic …

NON-ENZYMATIC GLUCOSE SENSING Glucose in saliva 20-70 µM Glucose in tears 100-300 µM

ENZYMATIC GLUCOSE SENSING Glucose in blood 3-8 mM



Non-enzymatic glucose sensing

Issues with non-enzymatic sensing 1) Sensitivity (bare Pt is too low, heavy

metals to enhance it: Pb, Bi, WO3 …) 2) Electroactive interference species 3) Resistance to chloride ions 4) Dissolution/Toxicity of heavy metals

Advantages over enzymatic sensing 1) Able to achieve continuous glucose monitoring 2) high stability compared to traditional glucose sensors 3) ease of their fabrication 4) Pain free …

K. Tian et al. / Materials Science and Engineering C 41 (2014) 100–118



Chemical bath deposition

Ingredients: 1) Nickel sulfate hexahydrate 2) potassium persulfate 3) ammonia solution Mix at room temperature Substrate: FTO or ITO covered UPILEX Immersion time: 5 minutes

Ni(OH)2 nanosheets

650

nm



30 35 40 45 50 55

Ni (211) Ni (200)

Ar90FG60

FG60FG30Ar90

α-Ni(OH)2 (110) α-Ni(OH)2 (111) α-Ni(OH)2 (200) NiO (111) NiO (222)

XRD

signa

l [ar

b. u

nits

]

Degree [°]

As prep

Ni nanofoam formation

100 200 300 400 5000.0

0.5

1.0

H co

nsum

ptio

n [a

rb. u

n.]

Temperature [°C]

As prep Argon

Temperature programmed reduction

BET surface area ~ 25 m2/g

Ni(OH)2 NiO NiO Ni nanofoam

350°C, Ar

350°C, FG



Glucose sensor

CV (0 – 0.8 V vs SCE) 30 cycles, at least

Y.Miao et al., Biosensors and Bioelectronics 53 (2014) 428–439

NiO NF



Glucose sensor tests

0 100 2000.0

0.3

0.6

0.9

1.2

50 µM

Cur

rent

den

sity

[mA/

cm2 ]

Time [s]

Ni nanofoam on FTO glass

20 µM

100 µM

0.0 0.1 0.2 0.3 0.40.0

0.3

0.6

0.9

1.2

cur

rent

den

sity

[mA/

cm2 ]

Glucose concentration [mM]

Glucose sensitivity:2.98 mA mM-1 cm-2

V = 0.5 Volt vs. SCE Response time ∼ 1 s

0.6

0.9

1.2

0.0 0.1 0.2 0.3 0.40.0

0.3

0.6

0.9

1.2

2

Gl t ti [ M]

Glucose sensitivity:2.98 mA mM-1 cm-2

Tears Saliva

K. O. Iwu, et al., Sensors and Actuators B (2015)



2D - conclusion

2D


Controlled, inexpensive growth of large surface area material ZnO NWs as ideal sensing nanostructure for pH NiO NF for non-enzymatic, high sensitivity glucose sensing

L. Maiolo, et al., Appl. Phys. Lett. (2014)

K. O. Iwu, et al., Cryst. Growth Des. (2015) K. O. Iwu, et al., Sensors and Actuators B (2015)



Acknowledgements

0D

1D

2D

Post-doc & PhD: S Cosentino, EG Barbagiovanni, R Raciti Staff: A Terrasi, M Miritello, TEM group SUPPORT: EU-NASCENT, IT-PON_PV&ENERGETIC Coll.: Brown Univ. USA, Bilkent Univ. Turkey

Post-doc & PhD: EG Barbagiovanni, V Strano Staff: G Franzò, R Reitano, TEM group SUPPORT: IT-PON_PLAST_ICs Coll.: Univ. Catania, TAU Israel, CU USA

Post-doc & PhD: K Iwu, V Strano Staff: G Fortunato, L Maiolo SUPPORT: IT-PON_PLAST_ICs Coll.: Univ. Catania


The key role of surface in semiconductor nanostructures for photovoltaics and sensing applications

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