Observation of negative differential resistance and electrical bi-stability in chemicallysynthesized ZnO nanorodsNandini Roy, Avijit Chowdhury, and Asim Roy

Citation: Journal of Applied Physics 115, 223502 (2014); doi: 10.1063/1.4882017 View online: http://dx.doi.org/10.1063/1.4882017 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of defect states in magnetic and electrical properties of ZnO nanowires AIP Advances 3, 042110 (2013); 10.1063/1.4801937 Green luminescence from Cu-doped ZnO nanorods: Role of Zn vacancies and negative thermal quenching Appl. Phys. Lett. 102, 111106 (2013); 10.1063/1.4798240 Vertically aligned ZnO nanorod grown by hydrothermal based chemical method on glass substrate AIP Conf. Proc. 1447, 421 (2012); 10.1063/1.4710059 Ultraviolet electroluminescence from ordered ZnO nanorod array/p-GaN light emitting diodes Appl. Phys. Lett. 100, 171109 (2012); 10.1063/1.4706259 Effect of annealing on the structural and luminescent properties of ZnO nanorod arrays grown at low temperature J. Appl. Phys. 109, 103508 (2011); 10.1063/1.3586243

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

Observation of negative differential resistance and electrical bi-stabilityin chemically synthesized ZnO nanorods

Nandini Roy, Avijit Chowdhury, and Asim Roya)

Department of Physics, National Institute of Technology Silchar, Silchar, Assam 788010, India

(Received 8 April 2014; accepted 25 May 2014; published online 9 June 2014)

Zinc oxide nanorods/p-Si heterostructures have been fabricated by depositing the chemically

synthesized ZnO nanorods on p-type silicon substrate. Heterostructure shows electrical bi-stability

and negative differential resistance (NDR) only at the beginning of the forward bias region, and these

phenomena have been explained with the help of energy band diagram. An explanation is proposed

for the origin of electrical bi-stability in light of the electric field induced charge transfer across the

junction, and the NDR phenomena could be attributed to interfacial traps and defect level that arises

due to oxygen and zinc interstitial vacancies. Room temperature photoluminescence measurement of

ZnO nanorods exhibits the emission peaks at about 466 nm and 566 nm which are attributed to

oxygen vacancies and Zn interstitials. A correlation between NDR and blue emission phenomena in

the ZnO nanorods due to defects states has been established. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4882017]

I. INTRODUCTION

Electric-field controlled bi-stable devices have aroused

extensive interests over the past several years due to their

unique advantages such as simple fabrication process, large

memory density, and lower power consumption.1–3 One of the

essential features in the non linear current-voltage (I-V) char-

acteristics is negative differential resistance (NDR), which

would allow further development in information processing

system.4,5 NDR in I-V characteristics of a device shows the

region where current decreases with an increase in applied

voltage and this phenomenon has attracted considerable atten-

tion due to their tremendous potential applications in low-

power memory and logic circuits.6–8 Various possible mecha-

nisms such as the formation of electric filament, electric

charge transfer, and charge trapping in the thin films or nano-

particles have been used to explain the electrical bi-stability

and NDR phenomena.9,10 The NDR effects were observed

recently by various researchers in single layer, multilayer, and

nanocomposites thin films based on organic and inorganic

materials.1,7 Such types of bi-stable devices are a subject of

recent interests and given more attention and further investiga-

tion to achieve practical applications are also necessary.

Zinc oxide is a promising II-VI semiconductor material

with a direct band gap of 3.37 eV.11 Due to its wide band gap

and large excitonic binding energy (60 meV at room tempera-

ture), ZnO has attracted much attention for many device

applications, such as transparent conductors, electro-optical

devices,12,13 gas sensors,14,15 thin film transistors,16 and varis-

tors and solar cells.17–19 Interestingly, ZnO nanostructures

also show a broad photoluminescence (PL) in the visible

region (470–550 nm).20 The visible emissions originate from

defects such as oxygen vacancy which are believed to be

located near the surface region.20–22 The PL emission energy

as well as the intensity not only depend on the size but are

also related to the morphology of the nanostructures.

Recently, studies about fabrication of bi-stable and NDR

hybrid devices based on ZnO nanorods (NRs) on single crys-

talline substrate and organic surface begun to appear in the

literature.23,24 Yang et al.25 reported electrical bi-stability and

NDR in single Sb-doped ZnO nanobelts/SiOx/p-type Si heter-

ostructured devices. Room temperature NDR based on a sin-

gle ZnO nanowire/CuPc nanofilm hybrid heterojunction has

been reported by Guo et al.23

In this work, we have synthesized ZnO nanorods by sim-

ple chemical approach and investigated the electrical bi-

stability and NDR phenomena in the heterostructures based

on ZnO nanorods/p-Si under dark conditions at room temper-

atures. Current–voltage (I–V) measurements were carried out

to study the electrical behavior of the device. A suitable

energy band diagram has been proposed to understand the

electrical bi-stability and NDR phenomena. PL spectrum

shows the blue and the green emission peak which arises due

to the radiative transition from the extended Zni to valence

band and from the conduction band to deep levels, respec-

tively. Such type of devices have potential applications in

the field of bi-stable memory devices for electronic and opto-

electronic applications.

II. EXPERIMENTAL DETAILS

To synthesize ZnO nanorods, Ammonium persulphate

(NH4)2S2O8, sodium hydroxide (NaOH), and zinc metal

powder (Merck chemicals) were used as precursors. A solu-

tion of ammonium persulphate was prepared by dissolving

0.038 mol ammonium persulphate in 20 ml de-ionized water.

This solution was added drop wise to a solution, which was

prepared by dissolving 0.191 mol of NaOH in 20 ml

de-ionized water. Mixture was again added drop wise to

another solution prepared by dissolving 0.9 gm zinc metal

powder in 10 ml de-ionised water with continuous stirring at

room temperature. The solution was stirred by magnetic stir-

rer for 22 h during which the color of the solution changed

a)Author to whom correspondence should be addressed. Electronic mail:

28.asim@gmail.com. Tel.: þ91 3842 224879.

0021-8979/2014/115(22)/223502/6/$30.00 VC 2014 AIP Publishing LLC115, 223502-1

JOURNAL OF APPLIED PHYSICS 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

from grey to milky white gradually. The final solution was

filtered, washed many times with de-ionised water, and then

dried at room temperature.

The phase and crystal structure of the sample were

studied by X-ray diffractometer (XRD) (Bruker AXS D8

Advance) using CuKa (40 kV, 40 mA) of wavelength

0.15418 nm in 2h mode. The morphology of the nanostruc-

tures was imaged by using field emission scanning electron

microscopy (Fe-SEM, JSM-6700F, JEOL). The crystalline

structure and lattice fringes of nanostructures were further

examined with a high resolution transmission electron micro-

scope (HRTEM, JEM-2010, JEOL, Japan operating voltage

200 kV). The nanocrystalline powders were dissolved in

methanol solutions for UV-vis spectral studies and PL studies.

An UV-visible scanning spectrophotometer (UV-2401 PC,

Shimazdu, Japan) was used to record the optical absorption

spectra of the sample at room temperature. A spectrofluorom-

eter (Fluoromax-4, Horiba Scientific) was used to record the

PL spectra of the sample.

To make thin films, freshly prepared solution was used

by dissolving synthesized ZnO powder in methanol. Using

this solution, thin films were prepared by precipitation tech-

niques on the single crystalline p-Si substrate for the electri-

cal measurements. To allow the evaporation of solvents from

films, films were kept at room temperature for 24 h and then

dried in oven. On the top of the thin films, circular shaped

aluminum (Al) electrodes (thickness 50 nm, area 0.25 cm2)

were deposited by thermal evaporation (model 12A4DM,

Hind Hivac, Bangalore, India) through shadow mask.

Electrical properties were recorded using semiconductor pa-

rameter analyzer (Keithley 4200 SCS).

III. RESULTS AND DISCUSSION

A. Structural characterization

Figure 1 shows the FE-SEM image of the synthesized

product consists of randomly oriented ZnO nanorods. The

length and the diameter of the nanorods are estimated to be

in the ranges 100–150 nm and 10–20 nm, respectively. Few

nanorods with larger diameter were also observed from the

FESEM image. The inset of Fig. 1 shows the FESEM image

of collection of nanorods at higher magnification. This dem-

onstrates that the simple chemical method can be used for

synthesis of ZnO nanostructures.

Figure 2 shows the x- ray diffraction pattern of the zinc

oxide nanorods. The result indicates that the rods have wurt-

zite structure with lattice constants a¼ 0.3250 nm and

c¼ 0.5205 nm.26 The diffraction spectrum matches well

with the hexagonal ZnO phase reported in ICDD Database

Card No. 36-1451. No diffraction peak from impurity phases

was observed in our sample, suggesting that the synthesized

nanorods are of good quality. The grain size calculated from

Scherrer equation is 13.55 nm. The Williamson-Hall plot for

the nanostructure is also plotted, and average grain size and

the strain calculated from the slope and intercept of the

straight line comes out to be 16 nm and 2.7� 10�3, respec-

tively. It may be noted that the rods have preferentially

strong orientation along the (101) direction.

To make further structural investigations of sample

through the TEM analysis, the synthesized ZnO powder was

dissolved in methanol solution and collected on carbon

coated copper grid. Figure 3(a) shows the TEM image of

well organized ZnO nanorods with length varies in the range

50–100 nm and diameter varies in the range 7–15 nm. The

selected area electron diffraction (SAED) pattern for ZnO

nanorods is displayed in Fig. 3(b), which shows the crystal-

line nature of the synthesized powder. The HRTEM image

as shown in Fig. 3(c) reveals individual nanorods having

well resolved parallel lattice fringes. Fig. 3(d) shows the

magnified image of the selected nanorod with an inter plan-

ner spacing of 0.272 nm, consistent with the distance

between the (002) crystal planes. HR-TEM observation indi-

cates that the preferred growth direction of the nanorods are

parallel to the [001] crystallographic direction (c axis) as

indicated by an arrow in Fig. 3(d). The SAED patterns as

shown in the insets of Fig. 3(d) also suggest that the ZnO

nanorods are of single crystalline structure with hexagonal

wurtzite phase. Energy dispersive analysis of x-rays (EDAX)

of the synthesized sample confirms that the nanorods contain

zinc and oxygen only as shown in Fig. 3(e).FIG. 1. Fe-SEM image of ZnO nanorods synthesized by chemical method.

Inset shows the same image at higher magnification.

FIG. 2. XRD pattern of ZnO nanorods grown by chemical method. Inset

shows the Williamson-Hall plot for the XRD pattern.

223502-2 Roy, Chowdhury, and Roy J. Appl. Phys. 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

B. Optical properties

In order to obtain the band gap energy of the synthesized

sample, UV-visible absorption spectra were recorded at

room temperature using UV-Vis spectrophotometer and are

shown in Fig. 4. It can be seen that the position of maximum

intensity is independent of concentration of ZnO in the

suspension. The calculated band gap (Eg) is found to be

3.28 eV, by equating band gap value with the wavelength of

the spectra at which the differential absorption intensity is

maximum.27 The observed band gap value of our sample is

less than that of bulk ZnO (3.36 eV for wurtzite phase). This

smaller value of band gap compared to bulk ZnO is

attributed to defect states originating from Zn interstitials or

oxygen vacancies.28,29

To study the presence of defects, room temperature pho-

toluminescence spectra of the synthesized ZnO nanorods

were recorded with excitation wavelength 325 nm for differ-

ent concentrations of the suspension and are shown in Fig. 5.

The spectrum shows two visible emission peaks centered at

466 nm and 566 nm corresponding to blue and green spectral

emission. It can be seen from the spectra that the emission

intensity at 466 nm increases with increase in concentration

of the suspension. The spectral content of the blue green

band is determined by the relative weight of the two lines.

We have calculated the ratio of intensity of two emis-

sion peaks (I1/I2) and plotted against different concentrations

of the suspension as shown in Fig. 5(b). Nonlinearity of the

curve shows that there is an increase in blue emission inten-

sity compared to green emission with increase in powder

content in the solution.

The energy band diagram to explain blue and green

emission in ZnO nanorods is shown in Fig. 6. We correlate

the presence of blue green emission peak with the presence

of Zni interstitials and singly ionized oxygen vacancies in

our sample. When the energy of incident photon is greater

than the band gap energy of ZnO nanorods, electrons are

excited to the conduction band. The energy gap between Zni

and valance band is 2.9 eV as reported by Lin et al.30 and

that between conduction band and Zni is 0.22 eV as reported

by Kroger and Bylander.31,32 In our sample, the blue emis-

sion energy is calculated to be 2.66 eV, which we attribute to

the transition from extended Zni to valance band.33 The elec-

trons from conduction band to extended Zni go for a nonra-

diative transition and then go to the valance band, thereby

giving rise to the blue emission peak. To explain the green

emission peak at 566 nm, most of the electrons which were

pumped to conduction band de-excite to the deep levels giv-

ing rise to green emission peak.

C. Electrical characterization

Figure 7(a) shows the dark current-voltage (I-V) charac-

teristics of the hetero-junctions, i.e., Al/ZnO/p-Si/Al device

for both sweep direction in the ranges from �2 V to þ2 V at

room temperature. Arrows in the Fig. 7(a) indicate the

sweeping directions. The device shows forward rectifying

characteristics with large NDRs at the beginning of forward

bias, i.e., when high positive voltage was applied to the p-Si.

The schematic diagram of the heterojunction is shown in the

inset of Fig. 7(a). The devices also show electrical bi-

stability in the forward bias direction.

FIG. 3. TEM images of synthesized ZnO nanorods dissolved in methanol so-

lution: (a) Shows nanometer size rods having different lengths and diame-

ters, (b) SAED patterns of the collection of nanorods; (c) a single nanorod;

(d) HRTEM image as well as a SAED pattern (inset) of the single nanorod,

confirming that the nanorod is of the single-crystalline; and (e) EDAX pat-

tern of ZnO nanorods.

FIG. 4. UV-visible absorption spectra of different concentrations [(1)

100 ll, (2) 150 ll, (3) 200 ll, (4) 300 ll, and (5) 400 ll] of ZnO nanorods

dissolved in 4 ml methanol solution. Inset shows the differential absorption

intensity versus energy plot.

223502-3 Roy, Chowdhury, and Roy J. Appl. Phys. 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

There are always two current values in the forward bias

direction. As sweeping the voltage from þ2 V to �2 V, the

current in the forward bias increases with the decreases of

the bias voltage. When the bias voltage reaches a certain

value, the current starts to decrease, forming an NDR region.

In the forward bias, I-V characteristics of the devices clearly

indicate the existence of high current and low current at the

same voltage. Such type of NDR effects and electrical bi-

stability were not observed when negative bias voltage was

applied to the p-Si during the sweeping from �2 to þ2 V.

For a given devices, the I-V characteristics are stable and re-

producible as was evidenced by the reproducible I-V curves

for multiple voltage loops. I-V characteristics of the devices

were also measured at different sweep rates, and the curves

are shown in Fig. 7(a). Result indicates that the voltages

(VNDR) at which the current starts to decrease at forward bias

depend on the sweep rates. Lowering the sweep rates, VNDR

shifts to the higher voltage region, and also the peak current

increases. NDR effects were also noticed at different voltage

loops as shown in Fig. 7(b). Such results suggest that NDR

effects occurred only at the beginning of the forward bias.

Also the maximum current, called the peak current (Ipeak)

attained by the devices at VNDR also depends on the voltage

loops for a fixed sweep rate. Figures 7(a) and 7(b) clearly

indicate that NDR phenomena and electrical bi-stability

become more prominent for large voltage loops and high

sweep rates.

FIG. 5. (a) Room temperature photoluminescence spectrum of different concentrations [(1) 100 ll, (2) 200 ll, (2) 300 ll, and (4) 400 ll] of ZnO nanorods dis-

solved in methanol solution. 1, 2, 3 & 4 corresponds in the graph from bottom. (b) Shows the variation of relative peak intensity with the ZnO concentration.

FIG. 6. Schematic diagram for the proposed mechanism of blue and green

emissions through extended Zni interstitials and deep level transitions,

respectively.

FIG. 7. (a) Shows the dark I-V characteristics of the heterojunction devices

in the voltage loops �2 to þ2 V at different sweep rates [1–4] and (b) shows

the IV curves at different sweep loops but same sweep rate.

223502-4 Roy, Chowdhury, and Roy J. Appl. Phys. 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

In order to demonstrate the mechanisms of electrical bi-

stability and NDR phenomenon, we consider the energy

band alignment of the devices under different biasing condi-

tions as shown in Figs. 8(a)–8(c). Figure 8(a) shows the

energy band diagram of the heterojunction at thermal equi-

librium. The band gap and the electron affinity values for

ZnO and p-Si are assumed to be Eg(ZnO)¼ 3.37 eV,

vZnO¼ 4.35 eV (Ref. 34) and Eg(Si)¼ 1.12 eV, vSi¼ 4.05

eV,35 respectively. For ZnO NRs, it is accepted that numer-

ous defect-related levels (including deep levels) normally

appear in the band gap,36 and the Fermi level of ZnO is near

the bottom of the conduction band. The defect-related levels

are generally attributed to the zinc interstitial and oxygen

vacancies (in bulk or on surface),37 surface states (of NRs),38

and/or interface states (between ZnO and p-Si).39 When ZnO

and p-Si are brought together into contact, ZnO serves as

electron donor and p-Si serves as acceptor. During the Fermi

level alignment, there is a transfer of electron across the

interface from the ZnO side to the p-Si side which causes the

accumulation of holes and electrons at the junction interfaces

of ZnO and p-Si, respectively.

These excess holes and electrons at the junction inter-

face leads to substantial band bending in both ZnO and p-Si.

Excess holes bend the conduction band of ZnO upward,

whereas the excess electrons bend the valence band of p-Si

downward at the junction interfaces.25 We have used the

Aluminum (work function 4.1 eV) as the top and bottom

electrode. It has been reported that Al form ohmic contact

with the p-Si.40 At thermal equilibrium, electrons from ZnO

side face a barrier of DEc¼ 0.3 eV and holes from the p-Si

side face a high hole barrier of DEv¼ 2.55 eV. Figure 8(b)

shows the energy band diagram of the devices under the

application of forward bias, i.e., when positive voltage was

applied to p-Si and negative voltage was applied to ZnO.

Application of forward bias shifts the ZnO bands upward

and p-Si band downward. Therefore, the electron and hole

injection barrier decreases significantly. In such cases, we

got very high current due to both types of carriers. In forward

bias conditions, electrons are injected into the conduction

band of ZnO, and holes are injected into the valence band of

p-Si from the metal electrode.33 At the very beginning of

applied forward bias voltage, the band bending significantly

decreases the barrier height for the carriers.41 As sweeping

the voltage from þ2 to �2 V, the current increases with

decreasing the voltage. After a while, these injected electron

and holes neutralized by the accumulated hole and electrons

at the ZnO and p-Si interfaces and the bending band become

flat as shown in Fig. 8(b) (dotted curve). In such cases, the

height of the potential barrier becomes higher, and the car-

riers face large potential barrier at the junction interface, and

the current decreases with decreasing the bias voltage, form-

ing a NDR region. Zinc interstitial vacancies also give rise to

extended donor states near the bottom of the conduction

band of ZnO as confirmed from the blue emission from the

PL spectra. At high forward bias voltages, these donor states

are completely ionized and donate large number of electrons

to the conduction band of ZnO which causes large current.

But sweeping voltage from �2 to þ2 V causes gradual exci-

tation of electrons from the Zni level that causes excitation

of electron to the conduction band. Few electrons might

combine with the hole accumulation layer near the ZnO

interface. Polarity dependent electrical bi-stability clearly

indicates that different current values are due to the excita-

tion of carriers from the defect levels.

Energy band diagram of the heterojunction devices

under the application of reverse bias is shown in Fig. 8(c). In

reverse bias, positive voltage was applied to ZnO, and nega-

tive voltage was applied to p-Si. As a result, ZnO band

shifted to downward, and p-Si shifted to upward. In reverse

bias, holes are injected into the valence band of ZnO, and

electrons are injected into the conduction band of p-Si. In

such cases, holes face a barrier of around 3.37 eV at the

Al/ZnO junction, and electron faces a barrier of 1.12 eV at p-

Si/Al junction. As a result, holes are totally blocked by the

FIG. 8. Show the energy band structure of the device at (a) thermal equilib-

rium, (b) forward bias, and (c) reverse bias, respectively. EV, EC, and EF rep-

resent the position of the conduction band, valence band, and Fermi level,

respectively.

223502-5 Roy, Chowdhury, and Roy J. Appl. Phys. 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14

Al/ZnO junction, but we got current due to electrons as the

electron barrier is around 1.12 eV. The bias polarity depend-

ence demonstrates that the NDR characteristics in our devi-

ces might be due to the interface effects,41,42 directly related

to the interfacial traps,43 leading to the formation of space

charge due to trapping

IV. CONCLUSIONS

In conclusion, large area distributed ZnO nanorods have

been prepared by using simple chemical method, and ZnO/p-

Si heterostructures have also been fabricated. NDR and elec-

trical bi-stability has been observed in ZnO nanorods/p-Si

heterostructures at room temperature. The origin of electrical

bi-stability is attributed to the electric field induced charge

transfer across the junction, and the NDR phenomena could

be ascribed to interfacial traps and defect level that arises

due to oxygen and zinc interstitial vacancies. Room tempera-

ture PL spectra of synthesized nanorods exhibit blue and

broad green emission at about 466 and 566 nm, respectively.

Possible origin of emission peaks has been explained with

help of energy band diagram invoking Zn interstitials and

deep oxygen vacancies level.

1L. P. Ma, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 2997 (2002).2J. Chen, W. Wang, M. A. Reed, A. M. Rawlett, D. W. Price, and J. M.

Tour, Appl. Phys. Lett. 77, 1224 (2000).3D. I. Son, C. H. You, W. T. Kim, J. H. Jung, and T. W. Kim, Appl. Phys.

Lett. 94, 132103 (2009).4D. Ito, T. Tomita, and T. Hatazawa, Appl. Phys. Lett. 90, 143118 (2007).5H.-K. Lee and M. H.-C. Jin, Appl. Phys. Lett. 97, 013306 (2010).6C. W. Chu, J. Ouyang, J.-H. Tseng, and Y. Yang, Adv. Mater. 17, 1440

(2005).7Y. Yang, J. Ouyang, L. Ma, R. J. Tseng, and C.-W. Chu, Adv. Funct.

Mater. 16, 1001 (2006).8A. Ramesh, P. R. Berger, and R. Loo, Appl. Phys. Lett. 100, 092104

(2012).9J. S. Jung, E. H. Cho, S. Jo, K. H. Kim, D. H. Choi, and J. Joo, Org.

Electron. 14, 2204 (2013).10S. Das, R. K. Singha, A. Dhar, S. K. Ray, A. Anopchenko, N. Daldosso,

and L. Pavesi, J. Appl. Phys. 110, 024310 (2011).11V. Srikant and D. R. Clarke, J. App. Phys. 83, 5447 (1998).12M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R.

Russo, and P. Yang, Science 292, 1897 (2001).13M. R. Singh, J. Appl. Phys. 106, 063106 (2009).14X. Wang, C. J. Summers, and Z. L. Wang, Nano Lett. 4, 423 (2004).

15S. Tian, F. Yang, D. Zeng, and C. J. Xie, J. Phys. Chem. C 116, 10586

(2012).16S. Y. Cho, Y. H. Kang, J.-Y. Jung, S. Y. Nam, J. Lim, S. C. Yoon, D. H.

Choi, and C. Lee, Chem. Mater. 24, 3517 (2012).17Y. Wang, X. Li, G. Lu, X. Quan, and G. Chen, J. Phys. Chem. C 112,

7332 (2008).18H.-M. Cheng, W.-H. Chiu, C.-H. Lee, S.-T. Tsai, and W.-F. Hsieh,

J. Phys. Chem. C 112, 16359 (2008).19D. C. Olson, S. E. Shaheen, R. T. Collins, and D. S. Ginley, J. Phys.

Chem. C 111, 16670 (2007).20A. van Dijken, E. A. Meulenkamp, D. Vanmaekelbergh, and A. Meijerink,

J. Lumin. 87–89, 454, (2000).21I. Shalish, H. Temkin, and V. Narayananmurti, Phys. Rev. B 69, 245401

(2004).22M. Ghosh, S. C. Gadkari, and S. K. Gupta, J. Appl. Phys. 112, 024314

(2012).23W. Guo, Y. Yang, J. Qi, and Y. Zhang, Appl. Phys. Lett. 97, 263118

(2010).24Z.-L. Tseng, P.-C. Kao, M.-F. Shih, H.-H. Huang, J.-Y. Wang, and S.-Y.

Chu, Appl. Phys. Lett. 97, 212103 (2010).25Y. Yang, J. Qi, W. Guo, Z. Qin, and Y. Zhang, Appl. Phys. Lett. 96,

093107 (2010).26S. Mandal, K. Sambasivarao, A. Dhar, and S. K. Ray, J. Appl. Phys. 106,

024103 (2009).27N. Roy and A. Roy, J. Mater. Sci.: Mater. Electron. 25, 1275 (2014).28R. M. Sheetz, I. Ponomareva, E. Richter, A. N. Andriotis, and M. Menon,

Phys. Rev. B 80, 195314 (2009).29S. A. Ansari, M. M. Khan, S. Kalathil, A. Nisar, J. Lee, and M. H. Cho,

Nanoscale 5, 9238 (2013).30B. Lin, Z. Fu, and Y. Jia, Appl. Phys. Lett. 79, 943 (2001).31F. A. Kr€oger, The Chemistry of Imperfect Crystals, 2nd ed. (North-

Holland, Amsterdam, 1974), Vol. 1.32E. G. Bylander, J. Appl. Phys. 49, 1188 (1978).33H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, and W. Cai, Adv. Funct. Mater.

20, 561 (2010).34M. Dutta and D. Basak, Appl. Phys. Lett. 92, 212112 (2008).35S. M. Sze, Physics of Semiconductor Devices, 2nd ed. (John Willy and

Sons, USA, 1981).36A. B. Djurisic and Y. H. Leung, Small 2, 944 (2006).37B. J. Jin, S. H. Bae, S. Y. Lee, and S. Imc, Mater. Sci. Eng., B 71, 301

(2000).38A. Layek, B. Manna, and A. Chowdhury, Chem. Phys. Lett. 539, 133

(2012).39F. Yakuphanoglu, Y. Caglar, M. Caglarb, and S. Ilican, Mater. Sci.

Semicond. Process. 13, 137 (2010).40Y. S. Ocak, J. Alloys Compd. 513, 130 (2012).41A. Chowdhury, B. Biswas, and B. Mallik, J. Nanosci. Nanotechnol. 13,

4134 (2013).42P. H. Nguyen, S. Scheinert, S. Berleb, W. Br€utting, and G. Paasch, Org.

Electron. 2, 105 (2001).43A. Sawa, T. Fujii, M. Kawasaki, and Y. Tokura, Appl. Phys. Lett. 85,

4073 (2004).

223502-6 Roy, Chowdhury, and Roy J. Appl. Phys. 115, 223502 (2014)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.217.138 On: Tue, 10 Jun 2014 08:03:14