Designing Defects: A few curious aspects of ZnO nanostructures

Joy MitraSchool of Physics, Indian Institute of Science Education and Research

ThiruvananthapuramSTS Forum 2016

Zinc Oxide• Direct Band gap ~ 3.3 eV

• Exciton Binding Energy ~ 60 meV

• Thermal conductivity ~ 500 W/m/K

• Refractive Index = 2.0041

• Wurtzite - Tetrahedral Structure

• Lattice constants a = 0.325 nm c = 0.52 nm

• As grown ZnO: n-type

• Research Challenge: p-type

ZnO: Optical Properties

• Emissions: UV ~ 390 nm

• Emissions: Broadband VIS centred ~ 550 nm

Absorption Spectrum Photoluminescence Spectrum

350 400 450 500 550 6000.0

0.4

0.8

1.2

1.6

2.0

2.4

Inte

nsit

y x

106 (

CP

S/µ

A)

Wavelength(nm)200 300 400 500 600 700 800

0.01

0.1

1

10

100

Abso

rptio

n (a

rb. u

nits

)

Wavelength (nm)

VO*

CBM

VBM

Energy Band Diagram

The origin of n-type doping ?

• Origin of n-type doping is rather controversial

• What can act like donors?

• 2 culprits that harbour donor electrons

• (1) Oxygen Vacancies (VO)

• (2) Zn Interstitials (IZn)

• But VO states are too deep in the Band Gap

• And formation enthalpy of Zn interstitials are too high ~ 4 eV.

PhotoluminescenceZnO

• Band edge emission is correlated with Green emission • Violet - Blue emissions correlated with Red emission • Red and Green/Yellow emissions anti-correlated

Emission - strongly dependent on Excitation

SEM Images of ZnO nanorods

• Band edge UV • Violet - Blue • Green - Yellow • Orange - Red

ZnO: Zn interstitials and O vacancies

IZn

IZn*

VO*VO

CBM

VBM

Energy Band Diagram

ZnO with Interstitial Zn and O vacancies• High surface to volume ratio ensures O vacancies

• Varying amount of interstitial Zn

Electrochemical Diameter ~ 40 nm Length ~ 400 nm

• Control (ZnO/ITO)

Oxidation of Zn foil + annealing Diameter ~ 400 nm Length ~ 4000 nm

• Zn Rich System ZnO/Zn

PhotoluminescenceZnO/ITO

• Band edge emission (375 nm) • Spectra strong function of λexc • Blue emission (400 - 470 nm) • Green emission (500 - 550 nm) • Orange/Red emission (600 - 700 nm)

ZnO/Zn

Emission - strongly dependent on Excitation

PL Excitation

• UV + Blue - Violet emissions ⇔ λexc ~ 325 nm

• Green emissions decrease monotonically with λexc

• Red emission ⇔ λexc > 380 nm

ZnO/ZnZnO/ITO

• Blue - Violet emissions ⇔ λexc > 380 nm • Red emission ⇔ λexc > 380 nm • UV ⇔ λexc < 380 nm • Green emissions undergo a transition with λexc

A modified band diagram

Electron band diagram and transitions evidenced from the PL/PLE spectra

Morphology of Zn rich ZnO nanorods Grain size distribution

• Individual Nano rods - not entirely single crystals • Stack of hexagonal crystallites • Top facets - decorated with grains • Side planes - disordered with facets

20 30 40 50 60 700.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

Per

cen

t [%

]

Maximum Caliber [nm]

• Range: 20 - 200 nm • Mode ~ 30 nm

40 60 80 100 120 1400

5

10

15

20

25

30

Per

cent

[%

]

Maximum Caliber [nm]

SEM and AFM Image

Conductance (dI/dV) Maps

• Modulate the DC Bias with a small AC signal ?

• DC Tunnel Current also becomes time dependent • (VAC)rms << VDC

typically (VAC)rms < 100 mV for |VDC| ~ 2 V.

VB

(t) = VDC

+ vo

sin(!t)

I(t) = IVDC + (

dI

dV)VDCvosin(!t) + (

d2I

dV 2)VDCv

2o

sin2(!t) + .....

Conducting Atomic Force Microscopy + Optical

fibre inputs illuminating the junction

The current signal oscillating at f = ω/2π is proportional to the local dI/dV

Conductance Maps - light and dark

• Small Grains ⬄ High Conductivity ⬄ High Photo Responsive

• Larger grains ⬄ Low Conductivity ⬄ Low photoresponse

Excitation: DARK 532 nm 355 nm

Topography

Conductance Maps - light and dark

Excitation: DARK 355 nm

Topography + CMAP

• Small Grains ⬄ High Conductivity ⬄ High Photo Responsive

• Larger grains ⬄ Low Conductivity ⬄ Low photoresponse

Conductance Maps: Dark — UV — Green

• Photoresponse for 355 nm excitation - spread over entire grain • PR for 532 nm - localised preferentially at the grain edges

— the most disordered regions

Topography + CMAPs + Line Scans

DARK 355 nm 532 nm

Transient Response

• Average τr decreases from 3s to 400 ms between 0.25 – 3 V

lowest detected value ~ 90 ms.

• τd1 has an average value of 2.5 ± 0.3 s without any bias dependence

• Slower τd2 decreases from 10 s to 7 s with increasing bias

 

 

 

Scientific Reports 6, 28468 (2016). RSC Advances, 5, 23540 (2015) Applied Physics Letters 100, 162104 (2012)

Negative PR in a ZnO device ?

• Resistance of this device Increases upon UV excitation • Resistance can be reproducibly controlled between 50 KΩ - 3 MΩ • The high resistance state is highly robust with decay times > 10 hrs

• Can the robust positive photoresponse of ZnO based devices be stymied or even reversed ?

IV Characteristics of device

• A Device of nanostructured n-type ZnO and p-type polymer PEDOT:PSS

Memory of High State

To Conclude ……

IISER Indian Institute of Science Queen’s University Belfast University of Surrey

Kingshuk Bandopadhyay K K Nanda Paul Dawson Ravi P Silva

Vijith Kalathingal S B Krupanidhi Sesha Vempati

Krishnanand Prajapati

Harikrishnan G

COLLABORATORS & FINANCIERS

email: j.mitra@iisertvm.ac.in webpage: http://jmitra.wix.com/joygroup

Research InterestsPLASMONICS

TUNNELLING INDUCED LIGHT EMISSION

OPTO-ELECTRONICS

ZNO GRO BASED SYSTEMS

ELECTRICAL TRANSPORT

NANOSCALE SCHOTTKY JUNCTION DEVICES

IMAGING BIOLOGICAL SYSTEMS

IN THE NANOSCALE

Phys. Rev. B, 94, 035443, 2016

Journal of Physics D, 42, 215101, 2009

Nanotechnology 20, 335202, 2009

Applied Physics Letters, 94, 233118, 2009

Scientific Reports 6, 28468, 2016

RSC Advances, 5, 23540 2015

Applied Physics Letters 100, 162104, 2012

Nanoscale Research Letters, 7, 470, 2012

Journal of Applied Physics 117, 244501, 2015

Journal of Physics: Condens. Matter 23, 422201, 2011

Journal of Physics D, 44, 125101, 2011

Nature Communications 7, 11665, 2016

email: j.mitra@iisertvm.ac.in webpage: http://jmitra.wix.com/joygroup