STS Forum 2016

1. Designing Defects:
A few curious aspects of
ZnO nanostructures
Joy Mitra
School of Physics, Indian Institute of Science Education and Research
ThiruvananthapuramSTS Forum 2016

2. 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

3. ZnO: Optical Properties
• Emissions: UV ~ 390 nm
• Emissions: Broadband VIS centred ~ 550 nm
Absorption Spectrum Photoluminescence Spectrum
350 400 450 500 550 600
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Intensityx106
(CPS/µA)
Wavelength(nm)
200 300 400 500 600 700 800
0.01
0.1
1
10
100
Absorption(arb.units)
Wavelength (nm)
VO
*
CBM
VBM
Energy Band Diagram

4. 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.

5. Photoluminescence
ZnO
• 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

6. ZnO: Zn interstitials and O vacancies
IZn
IZn
*
VO
*
VO
CBM
VBM
Energy Band Diagram

7. 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

8. Photoluminescence
ZnO/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

9. PL Excitation
• UV + Blue - Violet emissions λexc ~ 325 nm
• Green emissions decrease monotonically with λexc
• Red emission λexc > 380 nm
ZnO/Zn
ZnO/ITO
• Blue - Violet emissions λexc > 380 nm
• Red emission λexc > 380 nm
• UV λexc < 380 nm
• Green emissions undergo a transition with λexc

10. A modified band diagram
Electron band diagram and transitions evidenced from the PL/PLE spectra

11. 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 70
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
Percent[%]
Maximum Caliber [nm]
• Range: 20 - 200 nm
• Mode ~ 30 nm
40 60 80 100 120 140
0
5
10
15
20
25
30
Percent[%]
Maximum Caliber [nm]
SEM and AFM Image

12. 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 + vosin(!t)
I(t) = IVDC
+ (
dI
dV
)VDC
vosin(!t) + (
d2
I
dV 2
)VDC
v2
osin2
(!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

13. 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

14. Conductance Maps - light and dark
Excitation: DARK 355 nm
Topography + CMAP
• Small Grains ⬄ High Conductivity ⬄ High Photo Responsive
• Larger grains ⬄ Low Conductivity ⬄ Low photoresponse

15. 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

16. 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)

17. 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

18. To Conclude ……

19. 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

20. Research Interests
PLASMONICS
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