Realization of Diverse Waveform Converters from a Single Nanoscale

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Realization of Diverse Waveform Converter from a Single Nano-scale Lateral p-n Junction Cu2S-CdS Heterostructure Amit Dalui, Mrityunjay Pandey, Piyush Kanti Sarkar, Bapi Pradhan, Aastha Vasdev, Nabin Baran Manik, Goutam Sheet, and Somobrata Acharya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22131 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Realization of Diverse Waveform Converter from a Single Nanoscale Lateral p-n Junction Cu2S-CdS Heterostructure Amit Dalui1, Mrityunjay Pandey2, Piyush Kanti Sarkar1, Bapi Pradhan1, Aastha Vasdev2, Nabin Baran Manik3, Goutam Sheet2*and Somobrata Acharya1* 1

School of Applied and Interdisciplinary Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata -700032, India. 2

Department of Physical Sciences, Indian Institute of Science Education and Research (IISER), Mohali, Sector 81, S. A. S. Nagar, Manauli 140306, India. 3

Department of Physics, Jadavpur University, Kolkata, 700032, India.

*Corresponding authors: [email protected]; [email protected]

ABSTRACT: A differentiator is an electronic component used to accomplish the mathematical operation of calculus functions of differentiation for shaping different waveforms. Differentiators are used in numerous areas of electronics including electronic analog computers, wave shaping circuits and frequency modulators. Conventional differentiators are fabricated using active operational amplifier or using passive resistor-capacitor combination. Here we report that a single Cu2SCdS heterostructure acts a differentiator for performing numerical functions of input waveforms conversion into different shapes. When a rectangular wave signal is applied through tip of conductive atomic force microscope, a spike-like wave signal is obtained from the Cu2SCdS heterostructure. The Cu2SCdS differentiator is able to convert sine wave signal into a cosine wave signal and a triangular wave signal into a square wave signal similar to the classical differentiators. The finding of a nano-scale differentiator at extremely small length scales may have profound applications in different domains of electronics.

KEYWORDS: heterostructures, pn-junction, C-AFM, differentiator, waveform conversion. 1

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INTRODUCTION: Rapid miniaturization of electronics played an important role in many areas of human civilization. The evolution of miniaturization is driven by the idea of scaling of the device size. The proof-of-concept of nanoscale device fabrication is based on the use of individual nanocrystals (NCs) to realize diodes or transistors.1-8 The fabrications of devices using individual NCs opened up numerous possibilities toward realizing nanoelectronic circuits.9-15 While the signature current-voltage (I-V) characteristics provide a common basis of device performance, achieving more complex functions using an individual NC is indeed challenging till date. For example, performing mathematical operation of calculus functions for shaping different waveforms is often required in electronics, which is carried out by using a circuit called “differentiator”.16,17 A differentiator produces output voltage (or current) that is proportional to the rate of change of the applied input voltage. Thus, the output differentiated signal of square wave signal is a spike-like wave signal, differentiated signal of triangular wave signal is a square wave signal and differentiated signal of sine wave signal is a cosine wave signal. A passive differentiator consists of only resistors and capacitors (RC network) while an active differentiator includes some form of active component such as transistors or operational amplifier.16,17 Bulk heterostructures consisting of an interface of two different semiconductors have been widely used as an essential building block for advanced electronic devices.18-20 Nanoscale Cu2SCdS heterostructures have recently emerged as promising materials with multiple functional properties that find applications in nanoelectronics.1,21 Copper sulphide with composition Cu2-xS (0x 0.25) containing copper vacancies act as acceptor and sulfur vacancies in CdS act as donor forming a pn-junction in a single heterostructure.22-24 Here we report that a single Cu2SCdS heterostructure act as a differentiator circuit for input waveform conversion. When a rectangular wave signal is 2


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applied using the tip of a conductive atomic force microscope (C-AFM), a spike-like wave signal is obtained from the Cu2SCdS heterostructure. The Cu2SCdS differentiator is able to convert sine wave signal into cosine wave signal and triangular wave signal into square wave signal. Piezo force microscopy (PFM) measurements on a single Cu2SCdS heterostructure reveal piezoelectric and ferroelectric nature suggesting the presence of intrinsic capacitive effect in the heterostructure that is required for a differentiator action.

EXPERIMENTAL SECTION: Chemicals: All chemicals are purchased from commercial source and used without further purification. Synthesis procedures: Copper diethyldithiocarbamate [Cu(S2CNEt2)2]: Cu(S2CNEt2)2 precursor was prepared following reported method.25 In a typical procedure, 5 mmol Na(S2CNEt2).3H2O was dissolved in 100 mL of water in a beaker. In a separate flux, 2.5 mmol CuCl2 2H2O was dissolved in 50 mL water to obtain clear blue colour solution. Then, Cu2+ solution was added dropwise to the Na(S2CNEt2).3H2O solution under stirring to obtain dark brown precipitate. As obtained precipitate was then isolated by filtration and washed with H2O several times. Then, it was dried in air to get brown colour powder and stored for further use. Cadmium diethyldithiocarbamate[Cd(S2CNEt2)2]: Cd(S2CNEt2)2 was prepared following the same procedure using CdCl2 as Cd2+ source. A white colour product was obtained. Cu2-xS hexagonal plates: A solvent mixture consisting of 5 mL dodecanethiol and 5 mL trioctylamine (TOA) was degassed for 30 minutes at 120 °C. The reaction mixture was purged with nitrogen and heated to 240 °C. In a separate vial, a precursor solution was prepared by dissolving 0.1 mmol Cu(S2CNEt2)2 in 0.5 mL trioctylphosphine (TOP). This 3



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precursor solution was swiftly injected into the reaction solution at 240 °C and annealed for 10 minutes at the same temperature. Reaction was quenched by inserting the reaction flux into water bath and allowed the reaction solution to cool down to the room temperature. Cu2xS

hexagonal plates were precipitated by adding ethanol followed by centrifugation. Solid

precipitate was washed thoroughly with hexane and ethanol. Cu2SCdS heterostructures: A solvent mixture of 5 mL DDT and 5 mL TOA was degassed under vacuum at 120 °C for 30 minutes. The reaction mixture was purged with nitrogen and heated to 240 °C. A precursor solution of 0.1 mmol Cu(S2CNEt2)2 in 0.5 mL TOP was swiftly injected into the reaction solution at 240 °C and annealed for 10 minutes resulting a dark brown colour mixture. After annealing for 10 minutes, a cadmium precursor solution [0.05 mmol Cd(S2CNEt2)2 in 0.5 mL TOP] was rapidly injected into it and further annealed for 2 minutes. Then, it was cooled down to room temperature using water bath. Cu2SCdS heterostructures were precipitated using ethanol followed by centrifugation. Precipitate of Cu2SCdS heterostructures was purified by repetitive washing with hexane and ethanol for five times. Characterization X-ray Diffraction (XRD) pattern was measured on Bruker D8 advance powder diffractometer using Cu Kα radiation (1.5418 Å). Transmission Electron Microscopy (TEM) images were taken using a JEOL JEM 2010 electron microscope operated at an accelerated voltage of 200 kV. TEM samples were prepared by placing a drop of dilute solution of heterostructures in hexane on the surface of a carbon coated Au grid (300-mesh) and dried in air before measurement. Selected area electron diffraction (SAED) measurements were also performed with the same JEOL JEM-2010 microscope. Elemental analyses were carried out using X-ray photo-electron spectroscopy (XPS) measurements with an Omicron X-ray photoelectron spectrometer. For chemical mapping of heterostructure, a FEI, TECNAI G2 F30, S-TWIN 4


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TEM microscope operating at 300 kV equipped with a GATAN Orius B CCD camera was used. High-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) equipped with a scanning unit and an HAADF detector from Fischione (model 3000) is employed with the same microscope for chemical mapping of Cu2SCdS heterostructure. Chemical map the constituent elements were obtained by monitoring the Cu-K, Cd-L and SK edge respectively. Compositional analysis was performed by energy dispersive X-ray spectroscopy (EDX) attached to the TEM. The ferroelectric and piezoelectric functionality of individual Cu2SCdS heterostructure was demonstrated using piezo force microscopy (PFM) in contact mode operated in Dual AC Resonance Tracing (DART) mode.26 Measurements were carried out at resonance frequency in order to achieve maximum sensitivity and the resonance frequency was varied between 280 kHz to 300 kHz. Conductive atomic force microscopy (C-AFM) was carried out using PtIr coated tips on individual heterostructure deposited on ITO substrate.

RESULTS AND DISCUSSION: Cu2SCdS heterostructures were synthesized by partial cation exchange method where Cu1+ was substituted with Cd2+ ion.25 First, Cu2-xS hexagonal plates were synthesized using single source molecular precursor copper diethyldithiocarbamate [Cu(S2CNEt2)2] in a mixture of trioctylamine and dodecanethiol (Figure S1, Supporting Information). A precursor solution of cadmium diethyldithiocarbamate [Cd(S2CNEt2)2] in trioctylphosphine was injected to the previous mixture for cation exchange reaction (details in experimental section). Low resolution transmission electron microscope (TEM) image shows an average size of 80 ± 10 nm of the Cu2SCdS heterostructures (Figure 1a). Different segments of the heterostructures are visualized from the contrast difference, which originates due to the difference in atomic weight of the elements Cu and Cd present in the heterostructure (Figure 1b). 5



a

b

c Cu2S CdS

Cu2S CdS

5 nm

e

d

Height (nm)

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4 3 2 1 0 0

20

40

60

80

Distance (nm) Figure 1. Morphology and structure of Cu2SCdS heterostructures. (a) TEM image of Cu2SCdS heterostructures. (b) Higher resolution TEM (HRTEM) image of a single heterostructure showing average size of 80 ± 10 nm. (c) HRTEM image of the heterostructure showing lattice spacings’ of Cu2S and CdS components. Yellow arrows indicate the interface of Cu2S and CdS. (d) AFM topographic image of Cu2SCdS heterostructures. (e) Height profile of single heterostructure showing differences in the height between Cu2S and CdS components. The height profile is measured along the blue line marked in (d).

High resolution TEM (HRTEM) image shows well-resolved (100) and (800) lattice planes corresponding to bulk wurtzite CdS (JCPDS# 77-2306) and monoclinic Cu2-xS (JCPDS#340660) phases respectfully (Figure 1c).25 The fast Fourier transform (FFT) pattern of Cu2SCdS heterostructures supports the HRTEM observations (Figure S2, Supporting Information). Energy dispersive X-ray (EDX) analyses confirmed the chemical composition 6


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of the heterostructures to be ~2:1:2 of Cu:Cd:S respectively (Figure S3, Supporting Information). Powder XRD pattern of the Cu2SCdS heterostructures contains signature of both Cu2-xS and CdS segments (Figure S4, Supporting Information). XRD pattern matches with the bulk monoclinic Cu2-xS (JCPDS# 34-0660) and wurtzite CdS (JCPDS# 77-2306) crystal phases respectively suggesting a copper deficiency.25 Chemical mapping of the using HAADF-STEM technique reveals distinct locations of the Cu and Cd defining an interface within the Cu2SCdS heterostructure (Figure S5, Supporting Information). X-Ray photoelectron spectroscopy (XPS) measurements confirm Cu1+, Cd2+ and S2- oxidation states of the constituent elements of the heterostructures (Figure S6, Supporting Information). Surface topography of the heterostructure using AFM indicates a dimension 80 ± 10 nm which matches with the TEM observation (Figure 1d). Height profile extracted from the AFM image reveals a thickness less than 3 nm of the heterostructures (Figure 1e). Interestingly, the height profile reveals a sharp division indicating slightly larger thickness of CdS compared to Cu2S within the heterostructure. We have investigated the electrical transport property of a single Cu2SCdS deposited on ITO coated glass by using C-AFM. A rectangular wave voltage signal of amplitude 0.02 V and frequency 10 Hz was applied through the Ir-Pt tip and corresponding output current was measured simultaneously (Figure 2a and 2b). Interestingly, output current behaves in a completely different way in comparison to the input voltage. The output current show spikes at the rising and trailing edges of the input rectangular wave voltage representing a pulse wave current signal (Figure 2a and 2b). Such an input versus output characteristics can be obtained using a classical differentiator.16,17 A differentiator displays the output signal which is directly proportional to the rate of change of the input signal. The output voltage (Vout) of conventional operational amplifier based differentiator can be expressed as, Vout   RC

dVin , dt 7



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where Vin is the input voltage, C is capacitance and R is the resistance of the circuit (Figure S7, Supporting Information). A simple passive RC differentiator circuit is also able to convert the input waveform in a similar manner like operational amplifier based differentiator (Figure S7, Supporting Information). In both the cases, the input signal is applied to the capacitor of the differentiator. The capacitor blocks any DC content allowing AC type input voltage to pass through it. The output characteristic of the differentiator is dependant on the rate of change of the input signal.

Figure 2. Voltage versus current characteristics of Cu2SCdS heterostructure. (a) Input rectangular wave voltage signal. (b) Output pulse wave current signal. (c) The rising edge of a rectangular wave input voltage signal along with the positive output spike wave signal. (d) The trailing edge of a rectangular wave input voltage signal along with the negative output spike wave signal.

Since we have not used additional amplifier, our input versus output characteristics resembles simple passive RC differentiator circuit where the output current (Iout) follows the relationship, I out  C

dVin dV . There exists a stiff slope ( in ) in the rising-edge of a rectangular dt dt 8


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wave when a rectangular voltage pulse is applied as input signal (Figure 2c). As a result, the output current shows an instant spike which remains in-phase with the input voltage (Figure 2c). After the initial rising-edge, the input voltage reaches the peak value where the input voltage is constant implying no rate of change of input voltage (

dVin =0). Hence, output dt

current becomes zero and this condition remains so long input pulse remains unchanged. A rate of change of the input voltage occurs in the trailing-edge, when the input voltage pulse changes to negative saturation (Figure 2d). This change in the input voltage results in negative current spike at the output (Figure 2d). Thus, current spikes are obtained at the output depending on the rate of change of the input voltage in rising edge or trailing edge. The shape of the resultant output waveform of a conventional RC differentiator is determined by the time constant, τ = RC. Hence, these results clearly point out towards the presence of an intrinsic capacitive effect within the Cu2SCdS heterostructure.

Figure 3. PFM hysteresis loops. (a) PFM phase response versus voltage hysteresis loops in the voltage range 20 V to 50 V. (b) “Butterfly loop” in displacement versus voltage response indicating piezoelectric property of the Cu2SCdS heterostructure.

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In order to find out the origin of the capacitance of Cu2SCdS, we have carried out piezo force microscopy (PFM) in contact mode operated in Dual AC Resonance Tracing (DART) mode (characterization section).26 Applying a small ac voltage (Vac) riding on a dc bias voltage (Vdc) through the PFM tip onto a Cu2SCdS heterostructure leads to local structural deformation due to converse piezoelectric effect and the resulting strain from the heterostructure surface is detected by the PFM tip to generate a piezoelectric hysteresis loop. The phase versus voltage hysteresis loops obtained for Cu2SCdS heterostructure shows square-shape for all range of applied voltages (Figure 3a). A 180° domain reversal is observed due to switching of the direction of polarization along the direction of the applied electric field.27,28 which is a signature of the presence of ferroelectricity in the Cu2SCdS heterostructure. A coercive voltage in the range of 8-12 V is obtained from the phase versus voltage hysteresis curves (Figure 3a). Ferroelectricity in the wurtzite (P63mc) chalcogenides has been theoretically predicted by structural distortion from centrosymmetric (P63/mmc) by relative displacement of cations against anions along c-axis.27,28 In non-centrosymmetric wurtzite CdS, the cations and anions remain in tetrahedral coordination.1 Strain created on the basic unit by applying external electric field causes a polarization of the cations and anions resulting in piezopotenital inside the crystal.1 Electromechanical hysteresis loops are observed in the displacement (D) versus dc bias (Vdc) curves resembling the “butterfly loop” (Figure 3b). The loop displays piezoelectric response as well as polarization switching under an electric field. According to the converse piezoelectric effect, the strain is linearly proportional to the applied electric field. When the electric field is parallel to the polarization, the strain increases with the electric field and the maximum strain occurs at the highest electric field. Polarization switching in a ferroelectric subjected to an electric field leads electromechanical hysteresis loop. It is evident that every point on the hysteresis loop contains information about local piezoelectric deformation under 10


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applied voltage.29,30 Notably, factors like electrostatic and non-electromechanical effects may contribute to the hysteretic effects in PFM phase and displacement.31 To overcome these factors, all the measurements were carried out using switching spectroscopy piezoresponse force microscopy (SS-PFM) mode.32 In SS-PFM, Vdc is applied in a sequence of pulses instead of sweeping continuously and the measurement for the phase or the displacement was performed at the “off state” of the pulses. Additionally, topological image after the spectroscopic measurements shows no modification which could be attributed to the tipinduced electrochemical processes. Hence, we ruled out the possibility of the electrochemical effects on the PFM measurements. Observations of hysteretic phase switching and butterfly loops indicate that Cu2SCdS heterostructures possess ferroelectric and piezoelectric properties. Hence, Cu2SCdS heterostructures are fundamentally capacitors below the resonance frequency. Earlier reports suggest formation of pn-junction in a single Cu2SCdS heterostructures with p-Cu2S and n-CdS segments.1,33 Non-centrosymmetric CdS is a piezoelectric material with wurtzite structure while Cu2S is nonpiezoelectric material with a thinner thickness compared to CdS (Figure 1e).1 A charge depletion region is created due to the alignment of the Fermi levels at the interface between the p-Cu2S and n-CdS that induces a built-in potential in the heterostructure. An average built-in potential of ~250 mV was reported by electrostatic force microscopy measurement on a single Cu2SCdS nanorod.28 The charge density in Cu2S (1019/cm3) is larger than the charge density in CdS (1016/cm3).34 Hence, the contribution of Cu2S to the capacitance can be neglected because of the much larger charge density in the Cu2S section compared to the CdS section. Because of the polarization of ions in CdS by the applied voltage, the built-in potential in the Cu2SCdS heterostructure is affected, which can contribute to the overall capacitance of the heterostructure. Additionally, the piezoelectric charges developed owing to the strain under 11



external external electric field contribute to the built-in potential at the interface of the heterostructure.35,36 Since it is a new finding of a nano-scale differentiator, we further carried out electrical transport measurements using with different shaped input voltages to confirm the differentiation functions. In each case, the output current follows the rate of change of the input voltage establishing differentiator behaviour. When a sine wave is applied as input voltage (Figure 4a), output current cosine wave with a 90° phase shift is obtained (Figure 4b).

Figure 4. Voltage versus current characteristics of Cu2SCdS heterostructure. (a) Input sine wave voltage signal. (b) Corresponding output cosine wave current signal. (c) Input triangular wave voltage signal. (d) Corresponding output square wave current signal. (e) Rising edge of input triangular voltage waveform with frequency 25 Hz. The amplitudes of the triangular voltage waveforms are 12


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indicated in the inset. (f) Output square wave current response of the differentiator for input signal of 25 Hz. The colour codes are the same as shown in (e).

For triangular wave voltage signal (Figure 4c), the output current showed square wave signal (Figure 4d). For a triangle wave input, there are two steady ramps at the rising and trailing edges (Figure 4c). When a positive voltage ramp is applied, which is basically the rising edge of a triangular signal, the current rises very sharply for few milliseconds and become constant while the input voltage rises with constant slope (Figure 4d). The constant ramp input is converted into a flat dc output by differentiator action (Figure 4d). Similarly, the output current changes in the trailing edges of the triangular input signal resulting in square wave at the output (Figure 4d).

Furthermore, we varied the amplitudes and

frequencies of the input triangular voltages and measured the output current simultaneously. We applied input positive ramps with different frequencies (20 Hz and 25 Hz) and amplitudes ranging from 0 to 2 volts (Figure 4e and Figure S8, Supporting Information). Higher the voltage ramp per second, a higher dc output level of the square wave is obtained (Figure 4f). It is evident that the output square wave frequency increases by increasing input triangular wave frequency following the same trend of input waveform conversion (Figure 4e,4f and Figure S8, Supporting Information). These observations further support differentiator ability of the Cu2SCdS heterostructure for performing various numerical functions. We have calculated the time constant of the differentiator from the rising edge of the current versus time curves (Figure S9, Supporting Information). The time constant increases with increasing the amplitude of input voltages which is attributed to the increase in the capacitance of the Cu2SCdS heterostructure (Table S1, Supporting Information). The change in capacitance occurs due the fact that the change in input voltage changes the width

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of the depletion region of Cu2SCdS heterostructure due to its piezoelectric nature. This observation shed light on the origin of differentiator behaviour of Cu2SCdS heterostructure.

CONCLUSIONS: In summary, novel nanodevice concept in the form of differentiator using Cu2SCdS heterostructure is demonstrated here for the first time. The heterostructure is intrinsically a pn-junction containing a built-in potential at the interface, which originates required capacitance for the differentiator action. Ferroelectric and piezoelectric nature of the heterostructure suggests that the barrier potential can be modulated by external field. The differentiator converts rectangular waveform into spike-like waveform, sine waveform into cosine waveform, triangular waveform into square waveform and a constant ramp input to a flat dc output. The differentiator would be useful in converting square waveform into spikelike waveform considering similarity of rectangular and square waveforms. Shaping waveform using a nanoscale heterojunction is intriguing and holds promise of performing a variety of mathematical operations. The findings of the present report may open up new opportunities for strengthening miniaturized electronic devices that can surpass existing bulk devices.

ASSOCIATED CONTENT: Supporting Information Structural characterizations of Cu2-xS and Cu2S-CdS heterostructures; differentiator circuit properties and transient response time calculation etc.

AUTHOR INFORMATION: Corresponding Authors 14


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E-mail: [email protected] (G. S.) [email protected] (S. A.) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT: SERB Grant # EMR/2014/000664, DST, India, is gratefully acknowledged for financial support. P. K. S. acknowledges DST INSPIRE fellowship.

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Table of Content (TOC) graphic:

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Realization of Diverse Waveform Converters from a Single Nanoscale

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