Effect of temperature and micro morphology on the Ag Raman peak in nanocrystalline CuO thin films Shrividya Ravi, Alan B Kaiser, and Chris W Bumby, Citation Journal of Applied Physics 118, 085311 (201[.]
Effect of temperature and micro-morphology on the Ag Raman peak in nanocrystalline CuO thin films , Shrividya Ravi, Alan B Kaiser, and Chris W Bumby Citation: Journal of Applied Physics 118, 085311 (2015); doi: 10.1063/1.4929644 View online: http://dx.doi.org/10.1063/1.4929644 View Table of Contents: http://aip.scitation.org/toc/jap/118/8 Published by the American Institute of Physics JOURNAL OF APPLIED PHYSICS 118, 085311 (2015) Effect of temperature and micro-morphology on the Ag Raman peak in nanocrystalline CuO thin films Shrividya Ravi,1 Alan B Kaiser,1 and Chris W Bumby1,2,a) The MacDiarmid Institute for Advanced Materials and Nanotechnology, SCPS, Victoria University of Wellington, Kelburn Parade, Wellington 6140, New Zealand Robinson Research Institute, Victoria University of Wellington, P.O Box 33436, Lower Hutt 5046, New Zealand (Received 24 April 2015; accepted 16 August 2015; published online 28 August 2015) Raman spectra obtained from a nanocrystalline CuO thin film are observed to exhibit significant variation in the peak position and peak line-shape as a function of spatial position within the film We attribute this effect to variation in the degree of local heating beneath the focused spot of the Raman probe laser To understand this, we have undertaken a detailed study of the temperaturedependence of the CuO Ag Raman peak We observe a linear relationship between line-width and peak position, which persists over a wide temperature range, and is characteristic of a Raman process in which the temperature-dependence is dominated by anharmonic 3-phonon decay We provide an analytical description of the Raman line-shape as a function of temperature and use this model to interpret the degree of laser heating observed within our sample Using this relationship, we identify that the local micro-morphology of the CuO sample under study can dramatically affect the temperature achieved due to laser heating We find that spectra collected from the surface of “micro-bubbles” within the CuO film studied can reach temperatures of >1000 K beneath the C 2015 Author(s) All article content, except focused spot of our low power (5 mW) probe laser V where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4929644] I INTRODUCTION Confocal micro-Raman spectroscopy is a non-contact and non-destructive technique that is widely used as an analytical tool in the study of new materials The observed line-shape of a Raman peak obtained from an inorganic crystalline material can be influenced by a number of contributing factors including: phonon confinement,1 intrinsic stress,2 electron/hole-phonon coupling,3 and local heating.4–6 In particular, local laser heating can lead to significant red-shifts and broadening of Raman peaks obtained from nano-crystalline semiconductor materials Such materials can exhibit large absorption coefficients at the wavelength of the probe laser2 in conjunction with suppressed heat conductivities due to interface scattering mechanisms.7 This can result in substantially elevated temperature distributions under the probe laser, which may be either homogeneous4 or inhomogeneous.8,9 Cupric Oxide (CuO) is a crystalline semiconducting oxide, which has attracted continued interest due to the wide range of nano-morphologies,10–12 which can be formed using inexpensive and facile synthesis methods such as solution synthesis and thermal oxidation CuO has an optical bandgap in the visible region13 (1.35 eV at 300 K) and reported device applications of nanostructured CuO include infra-red photodetectors,14 gas sensors,15 catalyst surfaces,11 and field emission sources.16 Many authors17–26 have reported redshifts and broadening of the Ag Raman peak obtained from a) Author to whom correspondence should be addressed Electronic mail: chris.bumby@vuw.ac.nz 0021-8979/2015/118(8)/085311/7 various nanostructured CuO samples and have cited this as evidence of phonon confinement effects in their materials However, these reports rarely account for local laser heating of the CuO samples—which can give rise to similar effects upon the observed experimental spectra Given that CuO has a very strong absorption coefficient in the visible region (102 greater than silicon), laser heating cannot be lightly ignored However, there is very limited published data on the effect of elevated temperature upon the CuO Raman spectra, with earlier studies27,28 having largely concentrated on behaviour below 300 K In this work, we illustrate the potential for laser heating in CuO We first report a study of the temperature-dependence of the CuO Ag Raman peak at elevated temperatures up to 673 K, which we describe using the 3-phonon scattering model of Klemens.27 We have then used this model to investigate the impact of local laser heating upon this Raman peak and highlight the large variability in the observed degree of local heating, which occurs as a result of micro-morphological features which are present in our CuO thin film samples II EXPERIMENTAL A set of nanocrystalline CuO thin film samples were produced through a two-stage process First, a thin film of copper (thickness: 500–1000 nm) was deposited upon a sapphire (006) substrate by thermal evaporation under vacuum The Cu films were then placed in a tube oven and annealed at 500 C in air for h Initial sample characterisation has been carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD).29 Raman 118, 085311-1 C Author(s) 2015 V 085311-2 Ravi, Kaiser, and Bumby J Appl Phys 118, 085311 (2015) FIG Scanning electron microscope (SEM) images of the nanocrystalline CuO films studied in this work Images acquired at (a) 100 magnification and; (b) 30,000 magnification spectra were collected using a Jobin Yvon LabRam spectrometer with a 632.8 nm probe laser An initial power dependence study at a single spot on the CuO film was acquired using four different levels of incident laser power: mW, mW, mW, and 500 lW The laser power incident upon the sample was varied through using a series of neutral density filters, which were placed in the optical path between the laser and the microscope objective A series of spectra were then collected at temperatures between 200 and 700 K at 500 lW laser power and under a nitrogen gas atmosphere This was achieved using a sample heating stage located within a N2 gas-cooled cryostat A 100 (0.6 NA) objective was used for the cryostat measurements, whilst a 100 (0.9 NA) objective was used for the micromorphology study Both objectives have a beam waist at the focal point of the probe laser, which has been measured30 to be 0.6 lm III RESULTS A Sample characterisation Figure shows SEM images of a sample nanocrystalline CuO film at 100 and 30 000 magnification At high magnification (30 000), we observe that the nano-morphology of the film comprises tightly packed spherical crystallites with diameters between 40 and 60 nm.29 This morphology is a result of the thermal oxidation process used to produce our sample films, which drives a large increase in the film volume (The volume of the oxidised CuO film is 1.7 times larger than the volume of the initial copper film.) During this process, stress relaxation occurs both at grain boundaries and at the film surface Earlier XRD studies have confirmed that all the sample films produced for this study were monophase CuO and Williamson-Hall analysis indicated that the films were free of intrinsic stress29 at the macro-scale (using an XRD spot size of 1 cm) At lower magnification (100), the SEM image in Figure 1(a) reveals the presence of bubble-like morphological features in the film and we believe that these micro-bubbles also result from the expansion of the film during growth, in this case leading to stress relaxation through out-of-plane deformation of the film such that it becomes detached from the substrate Surface profile measurements of the sample using a Dektak profilometer indicated that these micro-bubble features vary in height from approximately lm to lm, with the regions between the micro-bubbles comprising a flat planar film, as is expected for a CuO film formed directly upon the flat underlying sapphire substrate B Temperature dependence of Ag mode Cupric oxide exhibits three Raman active modes27 (Ag, Bg1, Bg2), and when probed with an excitation energy of 1.96 eV at 300 K, the dominant signal31 is the Ag mode at 298 cm1 Figure shows the Ag Raman peak obtained from a nanocrystalline CuO thin film sample measured at room temperature under four different incident laser power levels We observe a clear power dependence of the peak position and line-width, whilst retaining a symmetrical peak lineshape This effect is most marked for spectra taken using a probe laser powers of more than mW, where xpeak undergoes a significant red-shift of up to 16 cm1 whilst the line-width broadens from cm1 to 22 cm1 However, for probe laser powers of 1 mW, the peak position is observed to remain approximately constant at 298 cm1, which is consistent with room-temperature values reported in the earlier literature.27,28 We shall show that the observed power dependence is caused by laser heating of the CuO thin film FIG Raman spectra of the CuO Ag peak collected at room temperature (293 K) from the nanocrystalline CuO thin film sample under different incident probe laser powers (5 mW, mW, mW, and 0.5 mW) 085311-3 Ravi, Kaiser, and Bumby In order to understand the extent of laser heating, we have first undertaken a temperature-dependence study in which laser heating effects were eliminated by using a low laser power (500 lW) with extended acquisition times, whilst the CuO sample was uniformly heated using an external high temperature heating stage Care was taken to ensure that all spectra collected were taken from sites on the sample located within an extended region of uniform flat planar morphology This was achieved by refocusing the laser beam and checking the morphology using the attached optical microscope prior to every measurement Figure shows the evolution of the Ag peak with increasing sample temperature This shows qualitatively similar red-shift and broadening behaviour to that seen in the power-dependent data The measured line-shape of the Ag peak comprises a linear combination of Gaussian and Lorentzian components, and across the range of temperatures measured we empirically find that the line-shape is welldescribed by a pseudo-Voigt function32 of the form given in the following equation: " !# " # 4ln2ịx xpeak ị2 C=2ị2 I0 I0 ỵ exp I x ị ẳ : x xpeak ị2 þ ðC=2Þ2 C2 (1) We have used this equation to define the two key temperature-dependent parameters, which are required to describe the Ag line-shape, namely, the peak position, xpeak and the line-width, C (C denotes the full-width at halfmaximum of Eq (1)) We obtained values for each of these parameters from each of our measured spectra by iterativeregression fitting of Eq (1) to the experimental data (using the Levenberg-Marquardt algorithm) as shown in Figure This approach enabled robust and unambiguous values to be assigned to xpeak and C for the individual spectra measured at each temperature Using the calculated values, we can FIG Plot showing Raman spectra of a CuO thin film sample acquired at temperatures from 198 K to 673 K at approximate temperature intervals of 25 K Note the red-shift of the Ag peak position and broadening of the peak line-width with increasing temperature Open circles show experimental data and solid red lines show fits to the experimental data using Eq (1) For clarity, spectra taken at each temperature have been uniformly offset along the y-axis The spectra marked “*” were taken at 298 K and approximately correspond to the 0.5 mW data shown in Figure J Appl Phys 118, 085311 (2015) then examine the relationship between the values of xpeak ðTÞ and CðTÞ obtained as the temperature of the sample varied, and this is shown in Figure There is a clear linear correlation between the peak position and line-width As discussed below, this is characteristic of a Raman peak for which the change in line-shape due to temperature is dominated by the temperature-dependent three-phonon anharmonic decay33 rate The Raman peak position of a crystalline solid, xpeak ðTÞ, is conventionally expressed as a function of temperature in the form of a linear combination of three terms3438 xpeak Tị ẳ x0 ỵ Dx1 Tị þ Dx2 ðTÞ: (2) The second and third terms on the right of Eq (2) describe the contributions from different effects that lead to a shift in peak position from the pure harmonic frequency of the optical mode, x0 The second term describes the contribution due to lattice softening from thermal expansion, which is expanded in Eq (3) Here, c is the Gruneisen parameter and aðTÞ is the temperature dependent coefficient of thermal expansion39 ðT (3) Dx1 ẳ x0 exp 3c aT ịdT : We can estimate the contribution from Dx1 ðTÞ to the peakshift in CuO using values from Ref 40 for cCuO 0.37 and aCuO ðT > 250 KÞ 6.0 106 K1 In this manner, we estimate the expected peak shift of the Ag line due to thermal expansion between 273 K and 673 K to be approximately 0.6 cm1 This is far lower than the experimentally observed shift of 18 cm1, which implies that in this case we can neglect the Dx1 term in Eq (2) The third term in Eq (2), Dx2 ðTÞ, describes the peakshift observed due to the anharmonic decay of Raman optical phonons into two or more phonons from other branches Following the approach of Klemens,33 Dx2 ðTÞ can be written as shown in Eq (4), which describes the contribution FIG Plot showing linear correlation between the measured line-width, CðTÞ, and the measured peak position, xpeak ðTÞ, for the CuO Ag Raman peak spectra obtained across the temperature range of 198 K to 673 K 085311-4 Ravi, Kaiser, and Bumby J Appl Phys 118, 085311 (2015) from the decay of an optical phonon into two identical phonons, each with frequency x20 and of opposite momenta (the so-called “three phonon” decay process) (4) Dx2 ðT ị ẳ Ax B1 ỵ C: hcx0 @ A 1 exp 2kB T The temperature dependence of the three-phonon process arises from the thermal occupation distribution of the daughter phonons, given by the Planck distribution: h i1 exp hcx 2kB T The observed broadening of the Raman peak with increasing temperature arises from damping of the excited optical phonon, and the line-width is inversely proportional to the phonon lifetime.41 The lifetime of the excited phonon is determined by its decay rate into lower energy phonons, which follows the same three-phonon process as described above Hence, CðTÞ can be described using an equation analogous to Eq (4), where only the independent fitting parameter is changed (from Ax to AC ) (5) CT ị ẳ AC B1 ỵ C: hcx0 @ A 1 exp 2kB T Both DCðTÞ and Dx2 ðTÞ follow the same functional form with temperature, leading to a direct linear relationship between xpeak and C of the form: xpeak T ị ẳ x0 ỵ Ax C T ị: AC (6) We expect this linear relationship to hold for all Raman spectra obtained from a uniformly heated sample in the regime where phonon decay dominates the temperature dependence of the Raman peak Using Eq (6), we find from Figure that Ax/AC ¼ 1.21 and x0 ¼ 307.1 cm1 for the Ag peak in our nanocrystalline CuO films In the high temperature limit where hcx 2kB T < 1, Eqs (2), (4), and (5) can be approximated to the form: xpeak x0 ỵ Ax ỵ 4kB Ax T hcx0 FIG Plot showing values of xpeak ðTÞ (circles) and CðTÞ (triangles) versus temperature for the CuO Ag peak measured in this work Dotted lines show fits to this data using Eqs (7) and (8), as described in the text C Impact of morphology on Raman signal The results of Section III B imply that if the substantial red-shift (16 cm1) observed in Figure is due solely to laser heating by the mW probe laser, then the CuO sample beneath the laser spot must experience a substantial rise in the local temperature To examine this effect in detail, we acquired datasets of multiple spectra from our CuO films at two different laser powers under ambient conditions (5 mW and 0.5 mW) A series of spectra were acquired at each laser power level by rastering the laser spot across the film surface, with spectra being collected at spacings of 10 lm Raman spectra acquired from each independent location were found to be symmetric about the peak position and hence were analysed by fitting to Eq (1), following the same procedure as used in Section III B Figure shows the values of xpeak ðTÞ versus CðTÞ obtained from the two datasets obtained using this method Also shown is the uniformly heated temperature-dependent data obtained in Section III B (Figure 4) (7) and C ðT Þ A C ỵ 4kB AC T AC ẳ AC ỵ bT: Ax hcx0 (8) For the CuO Ag peak, Eqs (7) and (8) are valid for T > 215 K Figure shows the values obtained for C and xpeak as a function of temperature, along with linear fits to the data points obtained at temperatures above 225 K From these fits, we obtain values of Ax ¼ 3.75 cm1 and AC ¼ 3.10 cm1 Using these parameters, we are now able to describe CðTÞ and xpeak ðTÞ across the entire temperature regime studied, and substituting these expressions into Eq (1) yields an analytical description of the Ag line-shape in CuO, which is valid up to temperatures of at least 673 K FIG Plot showing measured values of xpeak ðTÞ and CðTÞ obtained from spatially rastered sampling across the CuO film surface using different laser powers (0.5 mW and mW) Also shown are values obtained from the temperature dependence study (shown earlier in Figure 4) and the linear relation (dotted line) derived from that data 085311-5 Ravi, Kaiser, and Bumby Figure shows that data collected using the low laser power laser (0.5 mW) are tightly clustered around a single value of xpeak ¼ 29761 cm1, which is consistent with the 300 K value measured in Section III B However, at the higher laser power (5 mW), we observe a strong spatial variation in the measured values of both the peak position and line-width, with a difference of 22 cm1 between the highest and lowest observed values of xpeak We must exclude the possibility that this spatial variation is caused by material inhomogeneities that affect the local phonon modes, as we observe no variation in Raman spectra at the lower laser power In addition, such material inhomogeneities (e.g., impurities,42 defects,43 phonon confinement,44,45 surfacemodes46) would all be expected to give rise to an asymmetric Raman peak, which we not observe Similarly, homogeneous stress (beneath the probe laser) should be expected to lead to shifts in xpeak that are uncorrelated2 with C Again, this is not consistent with our observations From Figure 6, we also see that all of the data points obtained at the higher laser power agree closely with the linear relationship between xpeak and C obtained in the earlier external heating study (Figure 4) We take this as conclusive proof that the spatial variation must arise from differences in the local temperature arising from laser heating under the probe laser It is highly improbable that any alternative mechanisms could simulate such a reproducible match across the entire dataset One aspect to note is that this close agreement would not be observed if laser heating gave rise to a highly inhomogeneous temperature profile beneath the laser spot, as this scenario would cause substantial additional broadening of the measured peak.47 Instead, our results imply that all of the Ag line-shapes obtained at mW can be described by a single uniform apparent local temperature under the focused laser spot This situation is unusual, as the beam profile of the probe laser is generally expected to generate a thermal gradient beneath the focused laser spot,5,42,48 which drives heat flow via in-plane thermal conduction We speculatively suggest that a possible explanation in this case could be that photo-excitation by the probe laser causes a substantial increase in local carrier density within our CuO thin film This would then lead to an increase in the local thermal conductivity beneath the tightly focused laser spot, thus promoting temperature equalization within the region from which spectra are obtained We would expect this effect be most marked when the dark in-plane thermal conductivity beyond the laser spot remains extremely low, as is the case studied here Regardless of the underlying heat flow processes, the apparent uniform temperature beneath the laser spot can be determined from C (or xpeak ), using the parameters derived in Section III B (as shown in Figure 5) Figure shows that the linear correlation between these parameters persists even for values that lie well beyond those which were experimentally accessible using the external sample heating stage Extrapolating from Eq (7), we find that the lowest experimentally-observed values of xpeak (274 cm1) corresponds to a calculated maximum apparent local temperature that is in excess of 1000 K Figure shows a histogram of the same data points binned according to the apparent J Appl Phys 118, 085311 (2015) FIG Histogram of Ag Raman peak line-width showing the bimodal distribution of the line-width data obtained at full laser power The bottom axis shows the line-width converted to an effective temperature using the inverse of Eq (5) temperature The corresponding observed line-width is also plotted on the upper x-axis There is little variation in the observed line-width for the dataset obtained at 0.5 mW; however, two distinct clusters of spectra are observed in the high power dataset; a lower temperature cluster in the range 390 K to 500 K, and a higher temperature cluster between 800 K and 1050 K We believe that this bi-modal distribution reflects the underlying cause of the observed spatial variation in observed Raman spectra, which arises from the numerous micro-bubbles located within the flat planar CuO film (Figure 1) We observe that spectra collected from larger micro-bubbles (which are visible through the optical microscope) correlate with the histogram cluster between 800 and 1050 K, whilst spectra from the flat planar region correlate with values, which are consistent with the values clustered between 390 and 500 K The local temperature attained by laser heating can be affected by two key parameters that may vary with the micro-morphology of the film: (i) local rate of thermal conduction away from the laser spot and (ii) focal position of the laser relative to the sample surface In order to distinguish between the relative contributions from these two components, we conducted focal-length scans at a micro-bubble site and a planar site, in which the con-focal plane of the Raman microscope was scanned through the surface of the CuO film whilst acquiring spectra at a laser power of 5mW Figure shows the data obtained from this procedure, where an axial displacement of zero indicates that the laser is focused on the surface of the CuO film Negative axial displacement values indicate that the focal spot is located within the film The profiles obtained from the two sites differ markedly, with the maximum apparent temperature achieved at the micro-bubble site being 500 K more than at the flat planar site It should be noted that spectra taken at the two sites under the low laser power of 0.5 mW were identical; hence, the differing behaviour cannot be attributed to variations in local stress within the CuO film Rather, it appears that the local heat transport properties between the 085311-6 Ravi, Kaiser, and Bumby FIG Calculated temperature obtained from the observed Raman spectra as the confocal height of the Raman microscope was scanned through the sample surface at two different sites located at (i) a micro-bubble (crosses) and (ii) a flat planar region (circles) An axial displacement of zero denotes the laser was focused on the film surface and negative values denote the focal point was below the surface (i.e., within the film) two sites differ, presumably due to the fact that the CuO film has become physically detached from the underlying sapphire substrate at the micro-bubble site The absence of thermal coupling to the substrate will greatly reduce the rate of heat conduction away from the laser spot By comparison, the flat planar regions of the CuO film are strongly coupled to the underlying substrate and this depresses the maximum temperature, which can be attained by laser heating at these sites The dramatic reduction in the local thermal conductivity at a micro-bubble site is such that this effect continues to be observed even when the focal plane is displaced by >20 lm from the sample surface The data shown in Figure were acquired using a simple raster motion-control and as a result we would expect to see some variation in focal distance across the sample due to variations in the surface height of the CuO film The highlighted region in Figure corresponds to variations in surface height of up to lm, which is consistent with surface profile measurements of our samples (Section III A) The temperature values which lie within the highlighted regions closely match the two histogram clusters (at 390–500 K and 800–1050 K) shown in Figure As such, we infer that the observed bi-modal distribution of temperatures is a result of the different extent of laser heating experienced at microbubble sites and flat planar sites, respectively IV CONCLUSION The Ag Raman peak from our nano-crystalline CuO thin films exhibits a strong power-dependence, which is due to local heating effects caused by the focused probe laser We have carried out a detailed study of the temperaturedependence of this peak and shown that it can be fitted using a pseudo-Voigt line-shape over a wide range of temperatures We observe a linear relationship between the peak position and line-width, which holds for the entire range of temperatures studied here, and we note that this behavior is characteristic of any material in which the dominant J Appl Phys 118, 085311 (2015) temperature-dependent contribution to the Raman peak lineshape is due to anharmonic 3-phonon decay via the Klemens process We report 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CuO thin film are observed to exhibit significant variation in the peak position and peak line-shape as a function of spatial position within the film We attribute this effect to variation in the. .. can be influenced by a number of contributing factors including: phonon confinement,1 intrinsic stress,2 electron/hole-phonon coupling,3 and local heating.4–6 In particular, local laser heating