Statistically Analyzed Photoresponse of Elastically Bent CdS

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Statistically analyzed photoresponse of elastically bent CdS nanowires probed by light-compatible in situ high-resolution TEM Chao Zhang, Ovidiu Cretu, Dmitry G. Kvashnin, Naoyuki Kawamoto, Masanori Mitome, Xi Wang, Yoshio Bando, Pavel B. Sorokin, and Dmitri Golberg Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01614 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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Statistically Analyzed Photoresponse of Elastically Bent CdS Nanowires Probed by Light‐Compatible In Situ High‐Resolution TEM Chao Zhang*,†,‡, Ovidiu Cretu†, Dmitry G. Kvashnin§,⏊, Naoyuki Kawamoto†, Masanori Mitome†, Xi Wang&, Yoshio Bando†, Pavel B. Sorokin§, and Dmitri Golberg*,†,‡ †

International Center for Materials Nanoarchitectonics (MANA), National Institute

for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan ‡

Graduate School of Pure and Applied Sciences, Tennodai 1, University of Tsukuba,

Tsukuba, Ibaraki 3058577, Japan §

National University of Science and Technology “MISIS”, Leninsky prospect 4,

Moscow 119049, Russian Federation ⏊Emanuel

Institute of Biochemical Physics RAS, Kosigina st. 4, Moscow 119334,

Russian Federation &

School of Sciences, Beijing Jiaotong University, Beijing, 100044, P. R. China

ABSTRACT: We demonstrate that high resolution transmission electron microscopy (HRTEM) paired with light illumination of a sample and its electrical probing can be utilized for the in situ study of initiated photocurrents in free-standing nanowires.

ACS Paragon Plus Environment

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Morphology, phase and crystallographic information from numerous individual CdS nanowires is obtained simultaneously with photocurrent measurements. Our results indicate that elastically bent CdS nanowires possessing a wurtzite structure show statistically unchanged values of ON/OFF (photocurrent/dark current) ratios. Photocurrent spectroscopy reveals red shifts of several nanometers in the cut-off wavelength after nanowire bending. This results from deformation-induced lattice strain and associated changes in the nanowire band structure, as confirmed by selected area electron diffraction (SAED) analyses and density functional tight binding (DFTB) simulations. The ON/OFF ratio stabilities and photocurrent spectroscopy shift of bent CdS nanowires are important clues for future flexible electronics, optoelectronics and photovoltaics.

KEYWORDS: flexible optoelectronics, in situ TEM, ON/OFF ratio, photocurrent spectroscopy

Flexible electronics and optoelectronics have attracted considerable research interest in recent years due to the growing demand for lightweight electronic devices having high portability and low manufacturing cost, as compared to conventional bulk silicon technology.1,2,3 With large surface-to-volume and aspect ratios, high carrier mobility, and easily chemically decorated surfaces which could further be modified and functionalized, one-dimensional inorganic semiconducting nanostructures have proven to be key candidates for future flexible displays,4 lithium-ion batteries,5 supercapacitors,6 solar cells,7 generators,8 sensors,9 etc. One of the main existing challenges for such applications is the nanostructures’ electrical and mechanical


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operational stabilities. Even though some reports claimed that the nanowire conductivity is unchanged during mechanical deformations, there have been no in-depth

investigations

concerning

the

photoconductivity

or

photocurrent

spectroscopy of individual nanowires under conditions which would allow the direct visualization of deformation-induced lattice strain. Although these deformations are particularly important for flexible or stretchable devices,2 it is still unclear how structure changes in deformed nanowires affect their photoresponse. It is noteworthy that the latter was assumed to be unstable during elastic deformations.10,11,12 Here, we thoroughly address these issues by performing in situ HRTEM experiments using a specially designed optical TEM holder. We chose cadmium sulfide (CdS), a direct band gap semiconducting material widely used in diverse photoelectronic devices, as the test nanowire material.13 Using simultaneous electrical probing and HRTEM imaging, the currents induced in light-illuminated samples were measured by a source-measuring unit coupled with a light source, while at the same time allowing detailed visualization of bending deformation features. The first part of our results establishes a relationship between bending deformations and current values. In order to exclude the uncertainty introduced by the contact geometry (which is also an important factor for future flexible devices), and in order to account for the morphological diversity of the nanowires, we performed a careful statistical analysis of numerous sets of experimental runs. The second part of the work relies on photocurrent spectroscopy measurements, which showed red-shifts for the cut-off wavelength of the deformed nanowires that were considered to be a result of deformation-induced lattice strain and associated changes in the band structure. The third and final part of the work attempts to characterize the lattice strain and associated band structure changes using SAED analyses and DFTB simulations.


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CdS nanowires were synthesized via a chemical vapor deposition (CVD) method, as described in our earlier work.14 The typical size of nanowires tested was about 100 nm in diameter and a few micrometers in length. The crystal structure of the nanowires is wurzite, as confirmed by X-ray diffraction and SAED pattern, which are presented in Figure S1. The samples were firmly attached to a freshly-cut flattened gold tip using a thin layer of an electrically-conductive silver epoxy. As illustrated in Figure 1, the system comprises an optical fiber-compatible TEM specimen holder, featuring a piezo-tube which allows for a metallic probe (connected to a sourcemeter) and multimode optical fiber (connected to a light source) to be precisely positioned inside the pole piece of the microscope. The probe (with a tip radius between 50 nm and several micrometers) was aligned with the center of the optical fiber (with a 200 um diameter core) at a distance of about 0.5 mm. Probing, mechanical manipulations, HRTEM imaging and SAED studies were performed using an energy-filtering 300 kV JEM 3100FEF high-resolution TEM, under high vacuum (10-5 Pa) at room temperature. The specimen holders are an upgraded version of a piezo-driven optical TEM holder (“Nanofactory Instruments AB”), and a standard double-tilt TEM holder (JEOL). To better understand the electronic structure changes of CdS nanowires under bending, theoretical simulations were additionally carried out using Density Functional Tight Binding (DFTB) method, implemented in DFTB+ software package.15 The DFTB approach has been well suited and widely used for the accurate description of atomic structure, electronic and transport properties of various compound nanostructures which contain a large number of atoms in a considered unit cell (more than 1000). A complete DFTB parameterization for Cd and its interaction


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with S was proposed by Sarkar et at.16 Such parameterization led to the best possible description of periodicity, in particular, in the case of surfaces, nanowires and molecular systems based on CdX (X = S, Se, Te). This also displayed a good agreement with reference DFT data of elastic constants, cohesive energies and band structures.16 Two types of in situ experiments were performed. Photocurrent I-V measurements, described in Figure 1 (Scheme 1), test the current-voltage response of nanowires which are illuminated using a laser diode with a working wavelength of 488 nm. Photocurrent spectroscopy measurements, described in Figure 1 (Scheme 2), test the wavelength dependency of the electrical current which is generated inside the nanowire under light irradiation. The light source for photocurrent spectroscopy was a laser driven white light source, monochromated in order to carry out wavelength-selected measurements. In order to improve the signal-to-noise (S/N) ratio, a chopper and a lock-in amplifier were introduced in the measurement system for this case. Dark currents and photocurrents of the nanowires were measured by the sourcemeter before bending deformation, during deformation and after complete nanostructure recovery. While the majority of nanowires revealed stable contact properties during the deformations and recoveries, some nanowires deviated from this behavior and showed increased or decreased currents. Figure 2a illustrates a contact to a nanowire in its original non-deformed state. The average strain was defined as ε

⁄ , where d is a diameter of the nanowire and R is its radius of curvature.

These two parameters were measured by employing a curvature fit to the TEM images, as shown in Figure S2 for a representative nanowire. In order to achieve a strong and stable physical contact, the probe was slightly pressed towards the


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nanowire, as shown in Figure 2a. The probe was then moved left about 200 nm, resulting in a strain of up to 1.68%, as shown in Figure 2b. The nanowire was then recovered to its original state by retracting the probe, as shown in Figure 2c. For each state, I-V measurements were performed in both dark and illuminated conditions, as summarized in Figures 2d and 2e. In an attempt to get a better understanding of the relationship between the deformation states of the nanowires and their electrical and optical responses, numerous experiments were carried out for a comprehensive statistical analysis. The most important parameter is the light ON/OFF ratio (photocurrent to dark current ratio) of the nanowires. We define three ON/OFF ratios, corresponding to the original, deformed and recovered states as:

,

,

,

where “dk” and “ph” correspond to dark current and photocurrent, while “ori”, “def” and “rec” refer to the original, deformed and recovered states of the nanowire, respectively. As shown in Figure 3a, although the values vary from case to case, caused by probing-induced contact changes, the ON/OFF ratios are rather stable for each individual case. The statistical distribution of the ON/OFF values in Figure 3b confirms this trend on a wider scale and also allows us to estimate an average value of around 10. The results of stable ON/OFF ratios were most common; however, some data showed deviations. The reason is the fact that our setup has limitations with respect to the number of electrodes employed. With only two electrodes, contact resistance becomes an important uncertainty.17 The effects of this variable are however absorbed by our statistical analysis, where it is normally distributed. The


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independent nature of this variable with respect to the resistance of the nanowire itself allows their contributions to be separated, enabling us to observe the effects which are due to the intrinsic nature of our sample. In order to obtain additional information regarding the nanowires, we performed photocurrent spectroscopy and simultaneous HRTEM imaging. In Figure 4, the photocurrent spectroscopy results are displayed before and during a bending process which introduces a 1.1% elastic deformation. The nanowire photocurrent cut-off wavelength has a 7.3 nm red shift during bending: in the initial state, the edge wavelength was 521.8 nm; during bending, this value increased to 529.1 nm. Figure S3 shows more examples which feature similar red-shifts for the cut-off wavelength. Overall, for 1.68%, 0.75%, 1.59%, 1.21% and 3.36% strains, 1.2 nm, 5.4 nm, -0.6 nm, 0.7 nm and 5.5 nm shifts were recorded. We obtain an average value of 3.3±2.9 nm; although there is some deviation in the data, it shows that the effect is not limited to individual cases. The cut-off value of the photocurrent spectra is related to band structure, which determines the near-band-edge emission (NBE) of the material. Our observations are consistent with previous work performed by measuring the cathodoluminescence of CdS nanowires inside SEM, where the authors observed red-shifted emission for the NBE peak under strain, which is an indication for a decrease in the bandgap value, in agreement with our data.18 Evidence of the lattice strains which could have induced the band gap change can be found in the SAED patterns and theoretical simulations. In a separate control experiment, the in situ bending of a CdS nanowire supported on an amorphous carbon mesh was induced by causing damage in the supporting carbon film using the focused electron beam. The carbon film had then been deformed and the nanowire was bent


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accordingly. This control experiment allows us to use a standard double-tilt sample holder and to accurately align the nanowires along low index zone axes, and thus to get accurate crystallographic information which was not accessible while using a piezo-driven single-tilt setup. Figure 5a and 5b show TEM images of one CdS nanowire before and after its bending. The diameter was measured to be around 47.7 nm, and its bending radius was 2110.6 nm, leading to a strain of 2.26%. The insets in Figure 5a and 5b show the SAED patterns taken with a selected area aperture diameter of ~127 nm. The SAED patterns reveal the crystallographic information perpendicular to the [100] zone axis. By zooming in, the diffraction pattern of CdS nanowire after deformation shows radial spot broadening, which can be attributed to the geometrical changes of the lattice structure due to a strain.19 Then we developed a model of bent CdS nanowires having various degrees of curvature in accordance with the experimental deformation values. A simulated supercell of CdS nanowire (consisted of 1500 atoms of Cd and S) is illustrated in Figure 6a. In order to avoid the presence of dangling bonds and surface currents over the bent nanowires they were covered by a uniform layer of hydrogen atoms. For each bending strain the electron density of states (DOS) was calculated (Fig. 6b). It was found that with increasing strain the band gap value decreases (see Fig. 6b) which is caused by the shift of the top of the valence band and the bottom of the conduction band. The obtained band gap behavior agrees well with the earlier DFT calculations carried out within a small bending range (up to 2%).18 Decreasing of the band gap is directly associated with the experimentally observed red shift of the


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photocurrent response. In the inset of Fig. 6b the dependence of the band gap value on the bending strain is presented. By increasing the strain up to 4% the band gap monotonically decreases from ~2.4 eV to ~0.9 eV. Because of the rather low bending deformations and absence of temperature effects in calculations no structural defects appeared. Only direct surface reconstruction within the bent regions had been observed which could generate the small distortion (decreasing distance between the neighboring surface atoms) and further reduce the band gap. It should be noted that the present nanowires were deformed elastically, compared to the previous studies where they had undergone larger strains (>10%).20 Finally, it is worth mentioning that the averaged results presented here cannot accurately describe any single selected nanowire in terms of performance. The nanowires vary in regards to their morphology and structural properties, even within the same batch. There is an inherent contradiction between the need to accurately control the properties of every nanowire and the high-yield production required for making multiple devices at an industrial scale. From a statistical point of view, within this variety of structures, the nanowires studied here share common features with respect to their photocurrent-to-dark current ratio behavior during the deformation cycles. This is particularly advantageous for devices manufactured from a large amount of nanowires. Indeed, successful examples of making flexible transparent electrodes,21 flexible photodetectors,22 flexible supercapacitor electrodes6, lithium-ion batteries,5 LED arrays,23 solar cells,7 etc. out of these structures have already been reported. To summarize, we have successfully performed photocurrent measurements for elastically deformed CdS nanowires inside the HRTEM. Using in situ electrical


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probing and light illumination, we have characterized the electronic (dark, light-off) and optoelectronic (light-on) features of individual nanowires undergoing mechanical deformation. To make the data reliable in a view of future technological applications, a large variety of nanowires was tested, allowing for a statistical analysis of their properties. All nanostructures reveal very close photocurrent-to-dark current ratios (ON/OFF ratios) in original, bent and recovered states, with an average value of approximately 10. Photocurrent spectroscopy of several examples shows red shifts of the order of several nanometers for the photocurrent cut-off wavelength. According to the theoretical calculations, these small shifts are caused by deformation, which induces variations in the band structure. Our experiments reveal a variety of bending-induced effects for individual nanowires, while showing that from a statistical point of view the nanowires display common features in their response to deformation, making them suitable for future flexible optoelectronic applications.


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FIGURES

Figure 1. Schematic of the experimental setup used for photocurrent I-V measurements (Scheme 1, upper part), and for photocurrent spectroscopy (Scheme 2, lower part).


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Figure 2. Representative TEM images of the original (a), bent (b) and recovered (c) states of an individual CdS nanowire positioned between fixed Au (left hand side) and movable W (right hand side) electrodes; and a summary of dark current (d) and photocurrent (e) measurements at different stages of the bending-recovery process. Calculated strains are marked on the TEM image and I-V plots.

a)

Original Bent Recovered

b) 48

68

24 53

6

1

54

65

117 89

72 70 69

133 106 88 97 7101 881 80 0 90 111 85 71 17 7 120 222 130 120 19 27 42 2 49 55 61 132 84 91 30 30 3 789 14 36 44 119 102 02 02 99 34 4 75 98 89104 59 20 79 124 134 103 3 114 1 14 4 57 62 2 96 86 16 6 191116 3 64 108 1 08113 08 18 28 109 125 25131 25 768 76 127 7 100 95 123 3 94 9 4 105 63 87 9 135 3139 57 502 50 93 137 37 3 92 9 23 32 115 43 47 39 45 22 74 477 112 121 128 5 51 58 118 18 110 82 56 37 66 129 107 15 25 1 60 67 126 136 33 2 138 36 73 38 12 35 23 21 1 29

4 10 0 11

10

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13

100

ON/OFF ratio

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46

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Figure 3. Statistical distribution of the measured ON/OFF (photocurrent/dark current) ratios for 139 bending/recovery experiments undertaken on individual CdS nanowires. (a) ON/OFF ratio scatter; a gray region, where the majority of cases were documented, is drawn as a guide to the eye. (b) Statistical analysis of the data in (a); the lines represent Gaussian fits to the 3 histograms.

Figure 4. Photocurrent spectroscopy measurements performed on a representative individual CdS nanowire. The spectra have been fitted with logistic decay functions


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(solid lines); the regions of the curves corresponding to the symmetry point of each function have been extrapolated (dashed lines) in order to determine the intersections with the horizontal asymptote (dotted line). Insets show a schematic of the bending experiment (lower-left) and a low-magnification TEM image of the selected nanowire in contact with the W probe (upper-right). The values of the applied elastic strain and experimental red shift are marked.

Figure 5. (a,b) TEM images of the nanowire before/after bending on a TEM carbon grid using a standard double-tilt holder due to the electron beam irradiation of the supporting C segments. The insets show SAED patterns along the [001] direction from areas marked by red circles. Representative framed parts of the SAED patterns are zoomed-in in the lower-right parts of the panels.


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Figure 6. (a) Atomic model of the simulated CdS nanowire in plane, top and 3D views. The color code for constituting atoms is marked in the inset. (b) Electron density of states for CdS nanowires with various degrees of bending. The color code is presented in the left-hand-side inset. The Fermi energy is shifted to zero. The right-hand-side inset depicts the band gap dependence on the bending strain.

ASSOCIATED CONTENT Supporting Information. Additional content includes X-ray diffraction and SAED patterns of the sample, strain measurements and further examples of photocurrent spectroscopy of deformed CdS nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Authors to whom correspondence should be addressed. E-mail address: [email protected] and [email protected]


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ASSOCIATED CONTENT Supporting Information. Additional content includes X-ray diffraction, SAED patterns, strain measurements and further examples of photocurrent spectroscopy of deformed CdS nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Authors to whom correspondence should be addressed. E-mail address: [email protected] and [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The research was financially supported by the International Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan, University of Tsukuba. C. Z. and D.G. particularly acknowledge a Grant-in-Aid No. 26289244 (MEXT, Japan) for funding. O.C. is grateful to the


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International Center for Young Researchers (ICYS) of NIMS for financial support. P.B.S. acknowledges the Grant of President of Russian Federation for government support of young PhD scientists (MK-6218.2015.2). The theoretical calculations were funded by the Ministry of Education and Science of the Russian Federation (Increase Competitiveness Program of NUST MISiS No. K2-2015-001 in the frame of Mega-Grant award No. 11.G34.31.0061 and State task 11.1077.2014/K). D.G.K. and P.B.S particularly acknowledge funding from the K2-2015-033 project of NUST-MISiS. The authors are grateful to the “Chebyshev” and “Lomonosov” supercomputers of the Moscow State University for the possibility of using a cluster computer for simulations. Part of the calculations was made on the supercomputer cluster “Cherry” provided by the Materials Modeling and Development Laboratory at NUST MISiS (supported via the Grant from the Ministry of Education and Science of the Russian Federation No. 14.Y26.31.0005). The authors thank Prof. I. Abrikosov for granting computer time.

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Scheme 1

Sourcemeter

Au

Optical fiber

Laser LDLS & Monochromator

Inside TEM (Vacuum) f, θ Sourcemeter

Lock-in Amplifier


Scheme 2

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original

a)

bent 1.68% strain

b)

recovered

c)

500 nm e

s i

f2rs0sier f2rs0Oler f2rV0VOer rrs0sie rrs0Ole rrV0VOe

s V

e

e) ai

3 4 5 V s i s f r(A

V

5

4

3

o(dA

d)

V

rr(dA

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3 i o f1oa0aieo o f1oa02geo o f1os0s2eo o o oa0aieo o oa02geo o os0s2eo

3 ai a3

3 4 V s a i a f o( A


s

V

4

3

a)

b)

Original Page 23 ofBent 26 Recovered

Nano Letters 48

68

24 53

6 23 29 35 21

46

54

65 72 70 69

89 106

133

88 97 81 90 101 111 80 85 71 17 120 122 130 19 27 42 49 55 61 132 84 91 3 789 14 36 44 119 102 99 104 34 75 98 59 134 20 79 124 114 57 62 96 103108 86 16 113 28 116 125131 64 18 109 78 76 127 95100 123 94 105 63 87 93 135 52 50 137 139 92 32 115 43 47 39 45 128 22 77 74 112 121 5 51 58 118 110 82 56 37 66 129 107 25 15 1 60 67 126 33 2 136 138 73 38 12 4 10 11

26 31 30

41 40

ON/OFF ratio

117

13

100

1 2 3 4 5 6 710 8 9 10 11 12 1 13 14

83


1.8

Nano Letters

1.6

Page 24 10 ofμm26

Photocurrent (a.u.)

1.4

1 2 Original Bent 31 1.1 % strain 4 0.8 7.3 nm red-shift 5 0.6 6 0.4 7 CdS 8 Au 0.2 ACS Paragon Plus Environment 9 0 10 500 510 520 530 11 Wavelength (nm) deformation

1.2

540

Page 25 of 26

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nano Letters

b) 4% strain

Band gap, eV

a) DOS, arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

-Cd -S

2.5 2.3 2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0

1 2 3 4 5 Bending strain, %

0% 1% 2.5% 4%

-H

-3

-2

-1

0

1

Energy, eV


2

3

4

Statistically Analyzed Photoresponse of Elastically Bent CdS

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