grain size tuning of nanostructured cu2o films through vapour phase supersaturation control and their characterization for practical applications

Grain size tuning of nanostructured Cu2O films through vapour phase supersaturation control and their characterization for practical applications A Anu and M Abdul Khadar Citation: AIP Advances 5, 097176 (2015); doi: 10.1063/1.4932087 View online: http://dx.doi.org/10.1063/1.4932087 View Table of Contents: http://aip.scitation.org/toc/adv/5/9 Published by the American Institute of Physics AIP ADVANCES 5, 097176 (2015) Grain size tuning of nanostructured Cu2O films through vapour phase supersaturation control and their characterization for practical applications A Anu and M Abdul Khadara Centre for Nanoscience and Nanotechnology, University of Kerala, Kariavattom, Thiruvananthapuram - 695 581, Kerala, India (Received 12 May 2015; accepted 18 September 2015; published online 25 September 2015) A strategy for creating nanostructured films is the alignment of nanoparticles into ordered superstructures as living organisms synthesize biomaterials with superior physical properties using nanoparticle building blocks We synthesized nanostructured films of Cu2O of variable grain size by establishing the condition of supersaturation for creation of nanoparticles of copper which deposited as nanograined films and which was then oxidized This technique has the advantage of being compatible with conventional vacuum processes for electronic device fabrication The Cu2O film samples consisted of a secondary structure of spherical particles of almost uniform size, each particle being an agglomerate of primary nanocrystals Fractal analysis of the AFM images of the samples is carried out for studying the aggregation mechanism Grain size tuning of the nanostructured Cu2O films has been studied using XRD, and micro-Raman and photoluminescence spectroscopy C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4932087] I INTRODUCTION Optical and electrical properties of nanostructured semiconductors differ significantly from those of their bulk counterparts due to surface and quantum confinement effects, and the size specific properties of these materials render them suitable for potential technological applications To make use of the exquisite properties of nanostructured materials for practical applications, these materials are required in the form of nanograined films Nanostructured semiconductor films composed of thin layers of nanostructured objects such as nanoparticles, nanorods, nanowires or nanoporous networks with variable grain size and controlled porosity can exhibit properties superior to conventional thin films due to the deliberate engineering of nanoscale features into their structure and these films have practical applications in areas such as photovoltaics, electrochemical sensors, photoelectrochemical water splitting, biosensors and as antireflection layers.1–5 Techniques such as thermal evaporation,6 pulsed laser deposition,7 DC and RF sputtering,8,9 plasma evaporatihon,10 sol-gel technique,11 molecular beam epitaxy,12 electrodeposition13 and chemical vapor deposition14 have been employed for the synthesis of nanostructured thin films of different semiconductors Another technologically logical strategy for creating nanostructured films is the alignment of nanoparticle building blocks into ordered superstructures with non-resistive inter-particle contacts by bottom-up approaches and is one of the recent key topics in the area of nanomaterials.15 Nanoparticles may assemble into crystallographically aligned superstructures, mesocrystals,15 with multiple hierarchy levels and living organisms use nanoparticles as building blocks, instead of ions, for synthesizing several biomaterials such as bone16 with superior physical properties Synthesis of a variety of mesocrystals of metal oxides like TiO2,17,18 ZnO,19 Fe2O320 and CuO21 have been reported However, the artificial synthesis of nanocrystalline films through oriented assembling of nanoparticle building blocks remains a great challenge a Author to whom correspondence should be addressed Electronic mail: mabdulkhadar@rediffmail.com 2158-3226/2015/5(9)/097176/10 5, 097176-1 © Author(s) 2015 097176-2 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) Any vapor deposition method for synthesis of nanocrystalline films through assembling of nanoparticles should be capable of producing, initially, the required supersaturation for the generation of nanoparticle building blocks Recently, Shanid and Khadar,22 and Obey and Khadar23 have reported the preparation of nanostructured films of Cu2O and CuO, and ZnO respectively by the oxidation of nanostructured films of Cu and Zn deposited by thermal evaporation at a relatively high pressure of × 10−4 mbar of an inert gas The work in these reports is based on the generation of supersaturation in the deposition chamber, nucleation of nanoparticles and deposition of these nanoparticles on the substrates in the form of nanostructured films But the method for variation of supersaturation or the mechanism of nucleation of the nanoparticles and their aggregation to form finally the nanostructured films on the substrates have not been discussed In the present study, we have synthesized nanostructured films of Cu2O, which is the oldest p-type semiconductor with high absorption coefficient in the visible range making it an attractive material for application as low cost solar cells,24 of variable grain sizes and showing quasi mesoscopic characteristics through the oxidation of nanostructured Cu films deposited using vacuum thermal evaporation at relatively high pressures of an inert gas (nitrogen) Supersaturation of the inert gas in the deposition chamber with vapors of Cu resulted in the nucleation and growth of nanocrystals of Cu which deposited on the substrate as nanostructured Cu films The condition for supersaturation was varied either by varying the pressure of the inert gas keeping constant the mass of Cu evaporated or by evaporating different masses of Cu at a fixed inert gas pressure It was found that supersaturation established in the deposition chamber was determined by the pressure (p) of the inert gas and the mass (m) of Cu evaporated and the parameter (p/m) could effectively control the nucleation and size of nanograins of Cu which deposited on the substrate in the form of nanostructured copper films and which on oxidation produced nanostructured Cu2O films The Cu2O film samples consisted of a secondary structure of spherical particles of almost uniform size, each particle being an agglomerate of primary nanocrystals The primary and secondary particles showed clear increase in size with increase in the deposition pressure for a fixed mass of Cu evaporated and decrease in size with increase in the mass of Cu evaporated at a fixed pressure of inert gas The formation of the primary nanocrystals of different grain sizes depending on the supersaturation in the deposition chamber is explained on the basis of homogeneous nucleation theory The morphology of the nanostructued films of aggregated primary nanocrystals showed hierarchical, self-similar structure and fractal analysis of the AFM images is carried out for studying the aggregation mechanism HRTEM images showed lattice fringes and SAED patterns indicating oriented aggregation of the nanocrystals with quasi-mesocrystalline character UV-Vis absorption showed variation of band gap with grain size Photoluminescence spectra of the samples showed that the crystalline quality of the films did not alter with change in the grain size Micro-Raman spectra of the nanostructured films recorded for an excitation source of energy close to the band gap energy of the films showed resonance enhancement of the forbidden modes of Cu2O due to electron-phonon coupling The intensity of the forbidden Raman mode at ∼150 cm−1 increased conspicuously with increase in the grain size of the films confirming that the electron-phonon coupling strength in nanostructured films, in general, can be tuned by grain size variation which can efficiently be achieved by the vapour phase supersaturation control in the deposition chamber II EXPERIMENTAL DETAILS Nanostructured thin films of copper were deposited on glass substrates by thermal evaporation of metallic copper in a vacuum coating unit Glass substrates were cleaned by a standard procedure.22,23 Thermal evaporation of Cu was carried out by evaporation of a constant mass of Cu at different pressures of × 10−5, × 10−4 and × 10−4 mbar of nitrogen Films were also deposited by evaporating different masses of Cu for a fixed pressure of × 10−4 mbar On evaporation of copper, saturation and subsequent supersaturation of the nitrogen gas in the chamber with vapors of Cu lead to the formation of critical nuclei and their growth to form nanoparticles of Cu of different sizes, depending on the supersaturation, which deposited on the substrate as nanograined films The nanostructured Cu films were oxidized in air at different temperatures viz 230, 250 and 270 ◦C for 45 minutes The crystallinity, crystal structure and grain size of the film samples were investigated by GIXRD using Brucker AXS D8 advance X-ray diffractometer The average thickness of the film samples was determined using 097176-3 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) a Veeco Dektak-6M stylus profiler AFM measurements on the film samples were carried out using Digital Instruments Nanoscope E and with a Si3N4 100 µm cantilever having a force constant of 0.58 Nm−1 in contact mode HRTEM analysis of the samples was carried out using JEOL, JEM-2100 high resolution Transmission Electron Microscope The optical transmission spectra of the film samples were recorded using a JASCO V-650 double beam spectrophotometer and the photoluminescence measurements were carried out using Jobin Yvon Fluorolog-FL3-11 Spectrofluorometer The Raman measurements were performed at room temperature using a LabRam HR 800 Micro-Raman instrument with a10-mW Ar (488 nm) laser as excitation source III RESULTS AND DISCUSSION The GIXRD patterns of nanostructured Cu films deposited under different conditions and oxidized in air at different temperatures of 230, 250 and 270◦C for 45 minutes are shown in Fig S1 (SM).43 The peaks are not well formed for the samples oxidized at 230◦C The GIXRD patterns of the nanostructured films oxidized at 250◦C (SM)43 show a prominent peak at 2θ = 36.4◦ and two weak peaks at 2θ = 42.5◦ and 61.4◦ corresponding to the reflections from (111), (200) and (220) planes respectively of cubic Cu2O (ICDD File No.78-2076) A shoulder at 2θ = 35.6◦ and a weak peak at 38.8◦ are respectively due to the reflections from (-111) and (200) planes of CuO phase The intensity of the (200) and (220) peaks of Cu2O in Fig S143 was found to be decreased in comparison with their standard intensities (ICDD File No.78-2076) indicating a tendency for the alignment of nanograins of Cu2O in the [111] direction normal to the substrate The sizes of primary nanocrystals of the film samples were calculated using Scherrer’s formula25 making use of the FWHM of the (111) peak of Cu2O phase which is shown magnified for the different samples in Fig The grain sizes were respectively 10.36 nm, 11 nm and 13.52 nm for Cu2O films obtained from nanostructured Cu films deposited by evaporating the same mass of Cu (500 mg) at pressures of × 10−5, × 10−4 and × 10−4 mbar of nitrogen gas in the chamber (Table I) The grain sizes were respectively 12.36 nm and 13.4 nm for Cu2O films obtained by evaporating 400 mg and 300 mg of Cu at a pressure of × 10−4 mbar Under conditions of homogeneous nucleation, the size of the critical nuclei (r) and hence the size of the nanocrystals resulting from the growth of these nuclei should decrease with increase in the supersaturation according to the relation:26 r = 2γΩ/[kTln(C/C0)] (1) where γ is the surface energy per unit area, C is the concentration of the solute, C0 is the equilibrium concentration, k is the Boltzmann constant, T is the temperature and Ω is the atomic volume of Cu The variations of grain size of the films with pressure of the inert gas inside the deposition chamber and with mass of Cu evaporated are shown in Fig For a given mass of Cu evaporated, the supersaturation would increase with decrease in pressure of the inert gas, and for a given pressure of the FIG GIXRD patterns showing (111) peak of nanostructured Cu2O films synthesized by oxidizing at a temperature of 250◦C the nanostructured Cu films deposited by evaporating 0.5 g of Cu at pressures of (a) × 10−5 mbar, (b) × 10−4 mbar and (c) × 10−4 mbar, and by evaporating (d) 0.3 g and (e) 0.4 g of Cu at a pressure of × 104 mbar 097176-4 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) TABLE I p/m, thickness, grain size and fractal dimension data of nanostructured Cu2O films Pressure (mbar) p/m PaKg−1 Thickness (nm) Grain size (nm) Fractal Dimension Pore size (nm) Band gap energy (eV) × 10−5 × 10−4 × 10−4 × 10−4 × 10−4 16 20 25 33.33 40 195 146 140 135 127 10.36 ± 0.52 11 ± 0.38 12.36 ± 0.71 13.4 ± 0.8 13.52 ± 0.5 2.1286 2.0937 2.0714 2.0591 2.0303 45 54 61 64 73 2.5 2.49 2.48 2.45 2.43 gas, the supersaturation would increase with increase in the mass of Cu evaporated Figure indicates that size of the Cu2O nano grains decreased with decrease in pressure and with increase in mass of Cu evaporated, thus showing that the grain size decreased with increase in the supersaturation We found that the data shown in Fig can be unified by representing the supersaturation by the ratio (p/m) where p is the pressure of the inert gas in the deposition chamber and m mass of Cu evaporated Now, eqn (1) takes the form: r = 2γΩ/[kTln(p/m)] r = A/[ln(p/m)] (2) where A = 2γΩ/kT is a constant Figure shows the variation of the experimental values of grain size with 1/ln(p/m) Continuous line in Fig shows the fitting of eqn (2) to the experimental data points which confirm that the grain size of nanostructured Cu2O films could be tuned through vapour phase supersaturation control and the quantity (p/m) can replace the supersaturation (C/C0) in equation (1) The HRTEM image in Fig 4(a) shows lattice fringes of spacing ∼0.246 nm corresponding to the (111) lattice planes of nanoparticles of Cu2O indicating that the particles are perfectly crystalline The lattice fringes extend over a length much larger than the average size of the nanograins of the films pointing to a tendency for oriented aggregation of the nanograins in the direction [111] The selected-area electron diffraction (SAED) patterns of nanostructured Cu2O film (Fig 4(b)) show long range oriented alignment of the nanoparticle sub units in the films The high supersaturation produced in the deposition chamber in the present study favoured the nucleation of nanoparticles and particle mediated reaction pathways causing the formation of nanostructured films with orientational long range alignment of the nanoparticles, whereas low supersaturation should lead to the classical molecule based growth mechanism resulting in the deposition of thin films Oriented attachment of the nanoparticles is a consequence of the spontaneous self-organization of adjacent nanoparticle FIG Variation of grain size of nanostructured Cu2O films (a) Grain size variation with pressure of inert gas in the deposition chamber (b) Grain size variation with mass of Cu evaporated 097176-5 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) FIG Variation of grain size of nanostructured Cu2O films with 1/ln(p/m) building blocks in the vapour phase so that they share a common crystallographic orientation and this self-organization is a particularly relevant process in the nanocrystalline regime since bonding between the particles results in minimizing the overall surface energy.27 AFM images of the nanostructured Cu2O samples in Fig show that the surface texture and morphology of the films depend on the supersaturation in the deposition chamber determined by the inert gas pressure and mass of Cu evaporated Each of the Cu2O film samples consists of a secondary structure of spherical particles of almost uniform size, each particle being an agglomerate of primary nanocrystals of size in the range from ∼10.36 to ∼13.52 nm The secondary particles showed an increase in size with increase in the deposition pressure for a fixed mass of Cu evaporated and decrease in size with increase in the mass of Cu evaporated at a fixed pressure of the inert gas In the context of practical applications of nanostructured semiconductor films, the mechanism of formation of the primary particles and the type of aggregation of the secondary particles is important since the electrical current path in the films is determined by the inter-particle contacts We have shown above that the formation of nanocrystals from supersaturated vapor can be explained based on homogeneous nucleation theory of crystal growth and that the size of the nanocrystals is determined by the parameter (p/m) In the literature, the theory of growth of crystals from atomic scale aggregation has been relied upon to discuss the formation of stable secondary aggregates of primary nanostructures.28,29 Ballistic aggregation (BA), cluster-cluster aggregation (CCA), and diffusion limited aggregation (DLA) are the three models usually considered for explaining atomic FIG HRTEM image and SAED pattern of nanostructured Cu2O films synthesized by oxidizing nanostructured Cu films deposited by evaporating 0.5 g of Cu at a pressure of × 10−4 mbar 097176-6 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) FIG AFM images of nanostructured Cu2O films synthesized by oxidizing nanostructured Cu films deposited by evaporating 500 mg of Cu at pressures of (a) × 10−5 mbar, (b) × 10−4 mbar, by evaporating (c) 400 mg and (d) 300 mg of Cu at a pressure of × 10−4 mbar and by evaporating 500 mg of Cu at a pressure of × 10−4 mbar scale aggregation during crystal growth In the BA model, the particles are supposed to move along straight path until they get attached to the growing aggregate and direction of the growth is perpendicular to the substrate This kind of kinetics is typical for experimental situations when molecules move in a low density vapor as in the present case where the primary nanocrystals move in a space devoid of the solute vapors (since, the vapors have already been used up during the nucleation stage) Cluster-cluster aggregation involves the random motion of many particles, sticking together to form clusters which continue to perform random walks until all particles are part of one single aggregate The DLA model is based on single particles performing Brownian motion and is applicable under very low particle concentration We consider the formation of primary particles in the present study to be controlled by CCA and the aggregation of primary particles to be a process controlled by DLA Aggregation mechanism of nanocrystals can be appropriately discussed making use of fractal analysis.30 The fractal dimension, porosity and roughness of the film samples of the present study are analyzed by using Scanning Probe Image Processor (SPIP) software Determination of the particle size and pore size of nanostructured Cu2O films from AFM images revealed that both these quantities increased with increase in deposition pressure The fractal dimensions of the films (Fig 5) evaluated from the AFM images increased with increase in the value of p/m which represents the supersaturation (Table I) In the present study, all the film samples exhibited fractal dimension greater than It is known that the projection of a D-dimensional fractal structure in three-dimensional space onto a two-dimensional plane results in the same fractal dimension as in three-dimensional space when D is smaller than 2.0 and results in 2.0 when D is not smaller than 2.0.29 Making use of this result,29 we infer that the fractal dimensions of the aggregates constituting the nanostructured films of the present study are larger than 2.0 Although the fractal dimension does not directly correspond to the porosity, aggregated materials which have smaller fractal dimensions are reported29,30 to have higher porosity and hence a decrease in the fractal dimension may be considered to be qualitatively corresponding to an increase in the porosity.31 In the present study, the lower values of the fractal dimension of the nanostructured Cu2O films deposited at lower supersaturations indicate larger porosity in agreement with the larger measured values of porosity of these films (Table I) The optical absorption coefficient (α) of the films in the fundamental absorption region was calculated using the equation α = (ln T−1)/t where t is the film thickness and T the transmittance Cu2O being a direct band gap semiconductor, the band gap (Eg) was determined by plotting α2 against 097176-7 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) FIG PL spectra of nanostructured Cu2O films obtained by oxidizing at 250◦C the nanostructured Cu films deposited by evaporating (a) 0.3 gm, (b) 0.4 gm and (c) 0.5 gm of Cu at a pressure of × 10−4 mbar photon energy, hν (Tauc’s plot, SM Fig S3).43 The values of band gap energy of the Cu2O films deposited for different values of p/m was found to vary with the grain size of the films and are shown in Table I The band gap values obtained are in the range 2.43 eV to 2.5 eV (Table I) and are in close agreement with the values reported in the literature.32,33 The size dependent band gap is a quantum confinement effect and the observation of this effect for grains of nanostructutred films of Cu2O of size much larger than the Bohr exciton diameter of the material indicates the existence of a potential barrier inside the grains restraining the movement of the charge carriers as Noack and Eychmuller34 explained the blue shift of band gap in the case of ZnO thin films consisting of nanoparticles of size larger than Bohr exciton diameter Figure shows the photoluminescence (PL) spectra of nanostructured Cu2O films obtained by oxidizing the nanostructured Cu films deposited by evaporating different masses of Cu at a pressure of × 10−4 mbar and recorded with an excitation wavelength of 310 nm All the three samples show peaks at 350 (3.55 eV), 412 (3.01 eV), 425 (2.92 eV) and 513 (2.42 eV) These emissions are expected due to the excitonic transitions from the different sub levels of the conduction band (CB) to the Cu d-shells of the VB.35 The transitions 3d → 4s from the top of VB to the bottom of CB are forbidden according to selection rules Theoretically, it was predicted that transition probabilities from many bands below the top of VB to CB are negligible Valence Cu 3d band width is about eV and conduction bands 3d - 4s are almost 2.17 eV above VB 310 nm (4.007 eV) pulses may excite electrons from the Cu 3d band of VB to O 2p of CB since the transitions from Cu 3d of VB→ Cu 3d-4s of CB and Cu 3d-4s of VB→ Cu 3d-4s of CB are parity forbidden Quantum confinement of electrons in the nanoparticles gives rise to the splitting of energy levels with respect to bulk Thus the relaxations of the electrons occur from the different subenergy levels of O 2p band to Cu 3d band, which give rise to emissions at 412, 425 and 513 nm In the PL spectra of the samples in Fig 6, no significant shift in peak position, but only a small change in intensity (intensity being smaller for larger supersaturation) with change in the supersaturation (p/m value) was observed The variation in the grain sizes of the samples in the present study is not very much and hence an appreciable change in the intensity of the peaks and considerable shift in the peak positions may not be expected Figure shows the micro-Raman spectra of nanostructured Cu2O films synthesized by oxidizing at 250◦C the nanostructured Cu films deposited by evaporating 0.5 g Cu at pressures of × 10−4 mbar, × 10−4 mbar and × 10−5 mbar and recorded for an excitation wavelength of 488 nm The spectra show distinct Raman lines at ∼ 106, 150, 216, 298, 413, 495 and 641 cm−1 The line at ∼150 cm−1 which is close to the normally forbidden 1LO mode (153 cm−1) appears as the most intense line in the spectra The only allowed Raman phonon mode of Cu2O is observed at 495 cm−1 with a large frequency shift of ∼ 20 cm−1 compared to the literature value of 515 cm−1 The broad feature at 645 cm−1 with a frequency shift of ∼ cm−1 compared to the literature value of 640 cm−1 is attributed to the TO phonon mode and the less intense line observed at 296 cm−1 is attributed to the Ag mode of the CuO phase 097176-8 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) FIG Micro-Raman spectra of nanostructured Cu2O films synthesized by oxidizing at 250◦C the nanostructured Cu films deposited by evaporating 0.5 g Cu at pressures of (a) × 10−4 mbar, (b) × 10−4 mbar, and (c) × 10−5 mbar The electron-phonon coupling as a function of the size of nanostructures has been investigated by several authors.36,37 Theoretically, using a simple charge neutrality argument, Schmitt-Rink and coworkers predicted that the electron-LO phonon coupling mediated by Frohlich interaction diminishes with the decreasing size of nanostructures It is known that resonance enhancement of Raman cross-section in semiconductors is due to electron – phonon coupling.38 The electron–phonon coupling in nanoparticles is weakened with decrease in size due to the decrease in the density of states for both the electrons and the phonons, and also due to the increased overlap between the electron and the hole wave functions with decrease in size Enhancement of forbidden phonon mode with a pronounced resonance enhancement was reported by Martin et al for CdS.39 In ZnS nanoparticles, Saravana Kumar et al reported the resonance enhancement of 2TO mode which was not observed in bulk ZnS.40 Resonance enhancement of several theoretically forbidden Raman lines in Cu2O were reported in the literature and intraband Frohlich mechanism was suggested to be the mechanism for activating the forbidden longitudinal optical phonon mode at 153 cm−1.41,42 In the present study, the intensity of forbidden LO phonon at ∼150 cm−1 is resonantly enhanced and appears as the most intense line in the spectra It is found that the extent of resonance enhancement of the intensity of the ∼150 cm−1 mode which is determined by the electron-phonon coupling strength depends conspicuously on the grain size of the nanostructured film samples, intensity increasing rapidly with increase in grain size Thus the electron-phonon coupling strength in nanostructured films, in general, can be tuned by grain size variation which can efficiently be achieved by the vapour phase supersaturation control in the deposition chamber The development of technology for the synthesis of nanostructured semiconductor films of variable grain size is an important goal in nanoscience research since such films can exhibit properties superior to conventional thin films But the artificial synthesis of nanocrystalline films through oriented assembling of nanoparticle building blocks remains a great challenge Cu2O is a p-type semiconductor and is a promising low-cost material for photovoltaic and photocatalytic applications Nanostructured Cu2O layers on semiconductor and insulator substrates may exhibit interesting properties The present synthesis technique can be adopted for the deposition of nanostructured semiconductor films of variable grain size of not only Cu2O but also other semiconductors such as ZnO through assembling of nanoparticle building blocks and the method may be made compatible with conventional vacuum processes for electronic device fabrication IV CONCLUSION In conclusion, we have synthesized nanostructured Cu2O films of variable grain size by vapour phase supersaturation control and the variation of the grain size is explained using homogeneous nucleation theory The growth mechanism of the primary and secondary grains is discussed based on 097176-9 A Anu and M Abdul Khadar AIP Advances 5, 097176 (2015) fractal analysis Photoluminescence spectra of the films showed that the crystalline quality of the films did not alter with change in the grain size, which is a requisite characteristic of nanostructured films for practical applications The resonance enhancement of the intensity of the ∼150 cm−1 mode in the micro-Raman spectra of the nanostructured film samples showed that the electron-phonon coupling strength in these films can be effectively altered by controlling the grain size of the film samples The present synthesis technique can be adopted for the deposition of nanostructured semiconductor films of variable grain size through assembling of nanoparticle building blocks and the method may be made compatible with conventional vacuum processes for electronic device fabrication SUPPLEMENTAL MATERIAL GIXRD patterns of the nanostructured Cu films deposited by evaporation of 0.5 g of Cu at pressures of × 10−5, × 10−4 and × 10−4 mbar and oxidized at different temperatures viz 230◦C, 250◦C and 270◦C Plot of α2 as a function of hν for nanostructured Cu2O films synthesized by oxidizing at a temperature of 250◦C the nanostructured Cu films deposited by evaporating 0.5 g of Cu at pressures of × 10−5 mbar, × 10−4 mbar and × 10−4 mbar, and by evaporating 0.3 g and 0.4 g of Cu at a pressure of × 10−4 mbar ACKNOWLEDGEMENTS The authors thank Dr V Ganesan, Centre Director, and Dr.V.G Sathe and Dr V R Reddy, Scientists, IUC-DAE - CSR, Indore for providing facilities for AFM, micro Raman and GIXRD measurements The author M A K expresses his gratitude to KSCSTE for sanctioning him a fellowship under its Emeritus Scientist Scheme T.J Kempa, J.F Cahoon, S.K Kim, R.W Day, D.C Bell, H.G Park, and C.M Lieber, PNAS 109, 1407 (2012) A Yu, Z Liang, J Cho, and F Caruso, Nano Lett 3, 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