Distinguishing the Photothermal and Photoinjection Effects in

Sep 30, 2015 - Figure 1. Single crystal VO2 nanowires were grown using physical ... (a) The SEM image shows that the nanowires are 10s to 100s of μm ...
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Distinguishing the Photothermal and Photoinjection Effects in Vanadium Dioxide Nanowires Xi Wang† and Hanwei Gao*,†,‡,§ †

Department of Physics and ‡Materials Science Program, Florida State University, Tallahassee, Florida 32306, United States § National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States S Supporting Information *

ABSTRACT: Vanadium dioxide (VO2) has drawn significant attention for its unique metal-to-insulator transition near the room temperature. The high electrical resistivity below the transition temperature (∼68 °C) is a result of the strong electron correlation with the assistance of lattice (Peierls) distortion. Theoretical calculations indicated that the strong interelectron interactions might induce intriguing optoelectronic phenomena, such as the multiple exciton generation (MEG), a process desirable for efficient optoelectronics and photovoltaics. However, the resistivity of VO2 is quite temperature sensitive, and therefore, the light-induced conductivity in VO2 has often been attributed to the photothermal effects. In this work, we distinguished the photothermal and photoinjection effects in VO2 nanowires by varying the chopping frequency of the optical illumination. We found that, in our VO2 nanowires, the relatively slow photothermal processes can be well suppressed when the chopping frequency is >2 kHz, whereas the fast photoinjection component (direct photoexcitation of charge carriers) remains constant at all chopping frequencies. By separating the photothermal and photoinjection processes, our work set the basis for further studies of carrier dynamics under optical excitations in strongly correlated materials. KEYWORDS: Strong correlation, vanadium dioxide, frequency modulation, photoconductivity, photothermal, photoinjection, optoelectronic

T

Measuring the photoconductivity resulted from the photoinjection effects, i.e., the direct photoexcitation of free charge carriers through electronic transitions between the valence and the conduction bands in VO2,39 provides a direct approach to study charge carrier dynamics in devices. However, the conductivity of VO2 is considerably temperature dependent even at temperatures below the metal-to-insulator transition. The observed light-induced conductivity in VO2 has often been attributed to optical heating, also known as the photothermal effects.40,41 Nevertheless, little attention has been paid on the time scales of photothermal and photoinjection effects while studying the photoresponse of VO2 nanowire devices. In this work, we varied the chopping frequency of optical illumination to differentiate the fast response components from the slow response components. By this method, we managed to distinguish and separate the photothermal and photoinjection effects in strongly correlated VO2 nanowires. We showed that, at high chopping frequencies (>2 kHz), the photothermal effects can be highly suppressed in the AC component of the photoconductivity and, consequently, allow the photoinjection

he metal-to-insulator transition near the room temperature makes VO2 promising for applications such as smart windows,1 optical2−4 and electrical5,6 detectors/switches, thermal regulators,7,8 and Mott field-effect transistors.9,10 While it is expected to be conductive with half-filled valence band according to the conventional band theory, VO2 appears insulating at room temperature and experiences a sharp insulator-to-metal transition when heated moderately (∼68 °C).11−13 More interestingly, it has been reported that such phase transition can also be triggered by optical,14−16 electrical,10,17−24 and mechanical excitations,25−28 as well as hydrogen doping29 and orbital occupancy.30 Although reported repeatedly, the origin of the insulating phase in VO2 is still under debate.31−33 Prevailing theory proposed that a band gap in VO2 can be opened up because of the strong Coulomb interactions between electrons (Mott insulator) and the associated lattice distortion (Peierls insulator).34 Particularly, the photoinduced metal-to-insulator transition reported recently showed that the strong electron correlation dominates the formation of the Hubbard band and the Mott band gap in VO2.35 Materials with strong electron correlation may exhibit fascinating optoelectronic behaviors, such as the multiple exciton generations (MEG).36−38 © XXXX American Chemical Society

Received: August 4, 2015 Revised: September 16, 2015

A

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Figure 1. Single crystal VO2 nanowires were grown using physical vapor transport methods. (a) The SEM image shows that the nanowires are 10s to 100s of μm long and about 1 μm wide. (b) The AFM image shows a rectangular cross section of the VO2 nanowires. (c) The powder XRD pattern shows the VO2(M) insulating phase of the nanowires with a preferred growth orientation along [001]. (d) The real space HRTEM image of a VO2 nanowire shows clear lattices with d1 = 2.8 Å, d2 = 3.2 Å. Inset: the SAED pattern can be indexed to monoclinic P21/c with the zone axis [1̅21]. (e) The SEM image depicts a typical two-probe device defined with a 20 μm channel. (f) A sharp metal-to-insulator transition can be observed in the temperature-dependent resistance (R(T)) measured from the nanowire device.

is a strong evidence for the high crystalline quality and the desirable M1 phase of our VO2 nanowires.46 The small wiggling around 340 K in the R(T) curve is repeatable and likely caused by nonuniform substrate adhesion and stress induced by the end-to-end clamping by the contacts during heating and cooling.21,25,28 The photoresponse of the conductivity in our VO2 nanowires showed strong dependence on the chopping frequency of the optical illumination (Figure 2, symbols). The photoinduced change of the conductivity in the VO2 nanowires was measured using a chopped laser (CW Argon laser 457.9 nm) with a beam diameter of 60 μm (global illumination on the devices) and a power density of 6.7 W/cm2 (Figure 2 inset). To maintain the strong electron correlation, the measurements were carried out far below (220−270 K) the metal-to-insulator transition temperature in an environment controlled optical stage (Linkam LTS350), where the material was kept in the insulating phase. AC photocurrent representing the variation of the conductivity under the chopped optical excitation was collected using a preamplifier and a lock-in amplifier, with a reference from the chopper driver. At a fixed temperature, the AC photocurrent decreases significantly with increasing chopping frequency. Such decrease becomes minimal, and the photocurrent stabilizes at a finite value when chopping frequencies are >2 kHz. This phenomenon indicates that light-induced processes with different time constants exist in the measured photocurrent. To interpret the chopping frequency-dependent photoresponse, a theoretical model was developed with both the photothermal and photoinjection effects taken into account. The response time constant for photoinjection effects can be considered instant (typically on a sub-ns scale),35,47−50 while the photothermal response time is typically orders of magnitude longer limited by the heat dissipation rate.47,50−52

processes to dominate the photoresponse. Our results will allow for further study of the charge carrier dynamics under optical excitations in strongly correlated materials. The VO2 nanowires were grown using physical vapor transport methods (see Methods).42−45 The SEM images (Figure 1a) showed that most of the nanowires appear well crystallized with smooth facets. With a rectangular crosssection, the nanowires are typically 100−500 nm thick, 0.5−1.5 μm wide (Figure 1b), and up to hundreds of microns long. The crystal structure of the VO2 nanowires was confirmed using power X-ray diffraction (XRD) and transmission electron microscopy (TEM). All the peaks in the XRD patterns matched with the VO2 M1 insulating phase (Figure 1c). The dominating (011) and (022) peaks indicated the nanowires grew along a preferred crystal orientation [100].45 The selected-area electron diffraction (SAED) pattern taken along the zone axis [1̅21] further verified a monoclinic Bravais lattice in our VO2 nanowires (Figure 1d, inset). It is worth noting that moving the electron beam spot along the nanowires did not change the SAED pattern, indicating that the nanowires are all single crystalline, consistent with the faceted morphology of the nanowires in the SEM images. The corresponding TEM image with clear lattices fringes also shows nice crystallinity of the nanowires (Figure 1d). Single nanowire devices were fabricated on glass slides (Corning Launches EAGLE XG LCD Substrates, Resistivity 1013 Ω·cm). Two electrodes spaced by 20 μm were defined using standard photolithography techniques followed by thermal evaporation of 385 nm Cr/Ag thin films (35 nm Cr, 350 nm Ag) for Ohmic contact (Figure 1e). Temperature dependent resistance measurements (R(T)) were performed to examine the metal-to-insulator transition of the nanowires. A sharp metal insulator transition with >3000 times change in resistance can be observed at around 68 °C (Figure 1f), which B

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both the optical heating and the heat dissipation (cooling), whereas only the cooling is considered during the second half cycle (laser beam blocked). Each term in eq 2 can be ascribed as follows: (a) the photothermal heating rate is proportional to the absorbed laser power AP, whereas the heating efficiency is determined by x(T); (b) the cooling is proportional to the temperature gradient/difference between the nanowire and the heat sink (T − Ts), as well as the effective thermal conductance Keff of the device; (c) the rate of temperature variation in the nanowire is inversely proportional to the heat capacitance CVO2 of the VO2 nanowire. The model predicted that the transient photocurrent would deviate from a square wave because of the slow photothermal response (with respect to the chopping frequency), even though a square-wave chopped illumination was applied. Indeed, the transient response of photocurrent, recorded by a digital oscilloscope (Tektronix TDS3054), displayed a nonsquare waveform with 200 Hz chopping (Figure S2), which matches the prediction of our theoretical model (eqs 1 and 2). Good agreement was also obtained when we compared the experimental transient signal with our simulated waveforms of the photocurrent at other chopping frequencies (Supporting Information). The measured photocurrent is from the output from the lock-in amplifier, which is a time-averaged integral of the product of eq 1 and the trigonometric functions. The output can be expressed as the following in our model (Supporting Information):

Figure 2. AC photocurrent exhibits strong dependence on the optical chopping frequency. The measured (symbols) can be fit well using a two-component photoresponse model (solid lines). At various temperatures, the AC photocurrent decreases with increasing chopping frequency consistently. Inset shows the schematic of photocurrent measurement setup.

The theoretical model was developed based on the different time constants among the optical heating, the heat dissipation, and the photoinjection of charge carriers. The change of conductivity induced by the photothermal effects is explicitly a function of the temperature variation, whereas the photoinjection effects only have implicit dependence on the temperature (through properties such as carrier lifetime and the mobility). Therefore, the measured photocurrent originated from the photothermal and photoinjection effects are expected to be53 ⎧ AP eVbias ⎪ Iac(photoinjection) = γ(Ts) hυ w 2 ⎪ ⎪ τ(μ + μ ) ⎨ e h ⎪ ⎪ Iac(photothermal) = Vbiasσ(Ts) ΔE ΔT (t ) ⎪ kTs 2 ⎩

Iac =

π

2

1 2

( ) b(Ts) 2π

+ + f2

2 r(Ts) π (3)

Where the reduced parameters a(Ts) = Vbiasσ(Ts)

(1a)

b(Ts) =

ΔE x(Ts)A(Ts)P kTs 2 Keff (Ts)

Keff (Ts) C VO2(Ts)

(1b)

r(Ts) = γ(Ts)

A(Ts)P eVbias τ(μe + μ h ) hυ w2

(4)

The first term in eq 3 represents the photothermal induced photocurrent, a chopping frequency-dependent term; the second term originates from the photoinjected charge carriers, turning on and off instantly with the optical illumination and therefore chopping frequency independent. The measured data can be fit well using this model (Figure 2). In low frequency region, the temperature fluctuation of the device due to the laser chopping is around sub-mK. While in high frequency region, the temperature fluctuation is suppressed and the steady state temperature of the device is slightly higher than Ts (less than sub-mK). The results indicate that the photothermal component decays quickly as the chopping frequency increases. Comparing responses at different frequencies (Figure S2), the slow response of photothermal effects is gradually eliminated in the AC signal with increasing chopping frequency. When f > 2 kHz, the photoinjection effects become dominant and contribute to more than 95% of the photoresponse measured. In other words, the 2 kHz time limit for the photothermal contribution is consistent with the speed of thermal diffusion of the nanowire devices. That means, by increasing the chopping

where Vbias is the applied bias, Ts the heat sink temperature, A the absorption, P the laser power, and γ the external quantum efficiency (Supporting Information). σ(Ts) = σ0e−ΔE/kTs is the dark conductivity54 with activation energy ΔE = 0.19 eV from the R(T) experimental measurements. The photoinjection induced photocurrent (eq 1a) depends on the bias voltage Vbias applied on the of VO2 channel with width w, the carrier lifetime τ, and the mobility of charge carriers (electron mobility μe, hole mobility μh). According to the Fourier’s law,55 the instantaneous variation of temperature in the VO2 nanowire can be expressed as ⎧ dT [x(Ts)A(Ts)P − (T − Ts)Keff (Ts)] ⎪ = C VO2(Ts) ⎪ dt ⎨ ⎡ 1 1⎤ ⎪ dT − (T − Ts)Keff (Ts) = t∈⎢ , ⎥ ⎪ d t C ( T ) ⎣ 2f f ⎦ ⎩ VO2 s

a(Ts)b(Ts)

⎡ 1⎤ t ∈ ⎢0, ⎥ ⎣ 2f ⎦

(2)

where f is the optical chopping frequency and t is the time. Equation 2 indicates that the temperature variation in the first half period (laser beam on) includes the contributions from C

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Nano Letters frequency, we can isolate the photothermal contribution and to quantify the photoconductivity primarily contributed by the photogenerated charge carriers. Although the similar chopping-frequency dependence can be observed at various temperatures, the relative contributions from the photothermal and the photoinjection effects differ quantitatively. The AC photocurrent measured at low chopping frequencies contains both photothermal and photoinjection contributions as both processes can be considered fast with respect to the long chopping period. Alternatively, at high chopping frequencies, the photothermal responding time is comparable to the chopping period, and the photothermal induced signal is then highly suppressed as described in the theoretical model. The difference between the photocurrent measured at low and high frequencies can, therefore, represent the magnitude of the photothermal effects, whereas the highfrequency photocurrent approximately quantifies the photoinjection effects. By comparing the ratio(photothermal/photoinjection) = ratio(photocurrent(85 Hz) − photocurrent(2000 Hz)/photocurrent(2000 Hz)), we found that the higher the environmental temperature, the more the photothermal effects contribute to the overall photoresponse relatively (Figure 3a). Similar temperature dependence is also expected from the theoretical model (Supporting Information). The fitting results showed that the increased temperature results in higher photothermal efficiency (Figure 3a). The photothermal effect is proportional to the electron−phonon interaction constant G, which is also temperature dependent.56−58 When temperature is ascended, the electron−phonon scattering rate would increase in a nonlinear manner. Alternatively, the electron− phonon interaction is expected to be suppressed at cryogenic temperatures, which provides another means to reduce the photothermal component in the measured photocurrent. The effective thermal conductance Keff(T) should also be a function of the temperature. According the model fitting, Keff(T) grows with increasing temperature (Figure 3b). The thermal diffusion of the VO2 channel is determined together by the thermal conductivity of VO2 and the insulating VO2 and metal contact conductance. The laser generated heat likely dissipates through the nanowire channel and the metal electrodes. The electronic thermal conductivity of VO2 can be related to electronic conductivity by Wiedemann−Franz law11 Ke ∝ σT, which leads to an increase of the VO2 thermal conductance at higher temperature. The heat conductance (the inverse of Kapitza resistance) between the VO2 and the metal contacts interfaces is described by the acoustic-mismatch and diffuse-mismatch models.59−61 The increased temperature induces the different lattice expansion between VO2 nanowire and the evaporated Ag film (350 nm) interface. Consequently, the influence of temperature is on both the thermal conductance of VO2 and the contact interfaces, leading to the variation of effective thermal conductance of the devices. Compared with the literature values,7 the magnitude of temperature-dependent thermal conductance from our fitted results is reasonable. The agreement between the experiments and the theoretical model can also be found in the dependence of the photoresponse on the optical power density. According to eq S12, at a fixed temperature, the ratio between photothermal and photoinjection responses should not change with the intensity of the optical illumination. Such expectation is verified in the experiments. By varying the incident laser power (corresponding to AP in the model), we found that the normalized

Figure 3. Measured photoresponse is dependent upon the sink temperature but not incident laser power. (a) More contribution from the photothermal effects (represented by the photocurrent at 85 Hz minus photocurrent at 2 kHz) relative to the photoinjection effects (photocurrent at 2 kHz) can be obtained at higher sink temperature (black symbols). Similar results are observed from the fitting parameters based on the theoretical model as well (red lines). (b) The effective thermal conductance, extracted from the theoretical model fitting, appears to increase at higher sink temperature. (c) The magnitude of measured photocurrent is proportional to the laser power intensity (inset), while the normalized AC photocurrent follows the same frequency response. The measurements were carried out at 269 K.

photocurrent exhibits no dependence on the incident laser power (Figure 3c), indicating that the relative contributions from the photothermal and photoinjection effects remain the same. Not surprisingly, the magnitude of photocurrent is proportional to the laser power intensity (Figure 3c, inset). It is worth noting that all the optical excitation used here is considered weak, which does not trigger the metal-to-insulator transition in VO2 reported by others.39−41,62 The so-called persistent photoconductivity and the carrier trapping effect can also induce slow photoresponse. The relaxation time scale of persistent photoconductivity under Xray illumination was reported to be minutes to hours, much longer than what we are concerned about in our experiD

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ments.63−65 This phenomenon was sometimes attributed to photon/electron-induced metal-to-insulator phase transition, which is not likely to occur when illuminated by low-power continuous-wave light sources in the visible range.63,66 However, trapping and detrapping processes can cause photoresponses at the time scale within the range of which we are interested here (ms to μs). However, these processes, based on electron transitions between electron bands and surface/defect trapping states, tend to be suppressed at elevated temperatures. Since carriers have shorter trapping lifetime with higher temperature, the decreasing rate of the slower response with faster chopping should slow down with ascending temperature, leading to lower ratio (Is(85 Hz,T) − Is(2000 Hz,T)/Is(2000 Hz,T)).67,68 This is opposite to what we observed in the temperature-dependent measurements, indicating that such surface trapping effect is not dominant compared with the photothermal component in slow photoresponses (Supporting Information). In conclusion, we distinguished and quantified the photothermal and the photoinjection effects resulting in the photoconductivity of the VO2 nanowires simply by varying the optical chopping frequency. We found that selecting an appropriate chopping frequency of the optical illumination is critical for meaningful photocurrent measurements of VO2 nanowires. Particularly, by increasing the chopping frequency, the relatively slow photothermal process can be well suppressed, allowing the fast photoexcitation of charge carriers to dominate the photoconductivity. In addition, we found that the suppression of the photothermal component can also be assisted by lowering the temperature of the nanowires. This phenomenon can potentially be used to increase the external quantum yield of photoinjected charge carriers due to the reduced phonon-relaxation process, beneficial for efficient optoelectronics. Based on the agreement between the experimental measurements and the theoretical model we developed, we believe that the photothermal effects would only contribute less than 1% of the photoresponse when the chopping frequency is higher than 5 kHz, which paves the way for further studying the charge carrier dynamics under optical excitation in strongly correlated materials. Methods. Synthesis of VO2 Nanowires. The VO2 nanowires were grown using physical vapor transport methods.42−45 V2O5 powder was placed in an alumina boat at the center of a quartz tube furnace. The quartz tube was evacuated to a base pressure of 6.8 mTorr, while the pressure was maintained ∼5 Torr with a flowing Ar carrier gas during the growth. The reactions were carried out at 900 °C for 90 min. The nanowires were grown and collected on Si substrates (with a 200 nm thermal oxide layer) placed downstream with respect to the alumina boat.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Peng Xiong, and Prof. Biwu Ma for material synthesis and device fabrication, Dr. Eric Lochner for technical assistance with the electronic measurements, Mr. Jorge Luis Barreda and Mr. Timothy Keiper for the training of device fabrication. The TEM imaging was performed at the National High Magnetic Field Laboratory (NSF-DMR0654118) with help from Dr. Yan Xin. This work made use of user facilities in the High-Performance Materials Institute (HPMI), MACLAB (Chemistry), and Condensed Matter and Materials Physics (CMMP) at the Florida State University. This work is supported by the Start-Up Funds and the FirstYear Assistant Professor Award from the Florida State University.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03086. Derivation of the theoretical model; temperature dependence of photothermal to photoinjection ratio; temperature dependence of RC response and surface trapping effects; temperature dependence of pure photothermal power (PDF) E

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DOI: 10.1021/acs.nanolett.5b03086 Nano Lett. XXXX, XXX, XXX−XXX