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Thin semiconductor films: photoeffects and new applications
itk nwchWJn
Pergamon
Actq Vol 39.N. 8/9, pp IM-1236.1994 Copyright C 1994 Elrr,et Samuel Ltd Punted to Oral annum All nghn nuwed 0013-4686191$700 +nr
0013.4686(94)E0041-W
THIN SEMICONDUCTOR FILMS : PHOTOEFFECTS AND NEW APPLICATIONS FunsmxsA, • LARRY A NAGAHAIA, HAnhtE Yosmta, KA7suimeo Ante and K wir o HAsimtow Department of Synthetic Chemistry, Faculty of Engmeermg, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan AURA
(Received 7 September 1993) Abstract--Current topics in photochemistry and photoelectrochemlstry of thin semiconductor films are presented in regards to technological applications Examples include photoelectrochenical reduction of CO s , photocatalytic applications for deodorizahon, metalization via photochemical reactions, and the photochromic behavior of MoO 3 Key words semiconductor photoelectrochemistry, CO, reduction, photocatalyst, photochemical metalization, photochromism, Raman imaging 1. INTRODUCTION The interaction of light with semiconductor elect trodes has been studied for more than forty years, but it was not until the discovery of water photoelectrolysis at a TiO, electrode[1, 2] and subsequent the worldwide oil shortage in the 1970s that extensive research on photoeffects and semiconductor electrode research involve practical applications . During this period, solar energy conversion received the most attention as a way to solve the energy crisis However since that time, many other novel applications involving the use of photon energy with semiconductor electrodes have been reported For example, the need to solve certain environmental issues such as carbon dioxide reduction, deodorization/stenlization of public spaces and water purification have received much attention The main purpose of this paper is to present some recent applications of utilizing semiconductor thin films under the influence of light Specifically, we will discuss five areas of application (1) photoeleotrochemical reduction of CO, using p-type semiconductor electrodes, (2) photocatalytic materials for deodonzation and sterilization, (3) metalizahon on thin semiconductor layers, (4) visible-light induced photochromism in MoO3 , and (5) determination of the structural phase in photochromic materials using Raman imaging 2. SEMICONDUCTOR ELECTRODES FOR CO, REDUCTION Electrochemical carbon dioxide reduction has attracted renewed interest among researchers recently The reaction has been extensively studied over the years, however this renewed interest in this • Keynote lecture presented at the 44th ISE Meeting in Berlin
field originates from the following two reasons One is related to the so called "greenhouse effect" The other is the discovery by Hon et at [3] that carbon dioxide can be reduced to methane using a Cu electrode, with a current efficiency of almost 100% . When the electrochemical carbon dioxide reduction method is studied in relation to the "greenhouse effect", the problem of energy source should be taken into consideration Here, the electrochemical carbon dioxide reduction using light energy as the first energy source is presented The electrochemical systems using light energy can be classified into two categories . One is the convertbona] electrochemical system using a metal electrode equipped with a dry solar battery such as amorphous St In this system, solar energy is first converted to electric energy, then the electric energy is converted to chemical energy The other category is the photoelectrochemical system in which light energy is directly converted to chemical energy from a semiconductor electrode There have been many reports on the photoelectrochemrcal carbon dioxide reduction using various semiconductor electrodes such as GaP and ZnTe The main products reported thus far, however, are CO or HCOOH in aqueous solution Our intent is to obtain more valuable products by using photoelectrochenucal methods under milder cathodic conditions We have successfully applied two new p-type semiconductors as cathodes, namely copper oxide and silicon carbide, which selectively produce methanol from carbon dioxide and water We believe that this is the first reported case of electrochemically forming highly reduced products such as CH,, (eight electrons) and CH 3OH (six electrons) as main products with the aid of photon energy Copper oxide electrode The CuO electrode was prepared by the electrochemical oxidation of a pure Cu metal electrode . First, the Cu electrode was oxidized at a potential of
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Fig I Plots of the decomposition of acetaldehyde as a function of time for (a) high concentration and (b) low concentration Even at a concentration of a few prim, decomposition proceeds very efficiently under fluorescent light BL = black fluorescent light W L = white fluorescent light +0 3 V vs sce until 1 C of charge had passed At this
potential, Cu was oxidized to CuO Next, the CuO electrode was cathodically biased between a potential range of -01 to -04V vs sce in CO, saturated 0 1 M KHCO 3 aqueous solution A 500 W Xc lamp was used as a light source The main reduction product at this potential was CH, However, the CuO electrode was also reduced to Cu,O and then subsequently converted into Cu in this potential range Therefore, the reduction of CO, competes with the reduction of the CuO electrode itself and for this reason, the rate of CH, formation decreases gradually The highest current efficiency of CH, was initially obtained with about 60% at -0 3 V vs sce At this stage, no other reduction products were observed However, after prolonged electrolysis,
a White fluorescent light -~- Black light
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Irradiation time/h Fig 2 Plot of the fraction of MRSA bacteria surviving on a TI0 2 coated the vs time MRSA = methic lhn-resistant staphylococcus aureus
CH,OH was also detected with current efficiencies reaching 20% This presumably occurred due to the increased formation of Cu 2O on the electrode SIC electrode Single crystal and a microcrystalline p-type SIC were used as the electrodes in the present study In the case of single crystal SIC, the photocurrent was very small and hydrogen production dominated the reaction Hence we could not detect any reduction products of CO 2 In contrast, a microcrystalline SIC film electrode, which was deposited on indium tin oxide (ITO) by photoassisted (mercury lamp light) chemical vapor deposition (CVD) gave a considerable amount of reduction products of CO2 Although H, was still produced with a current efficiency of 80%, the rest of the electrons were used for the reduction of CO 2 Moreover, it should be stressed that the main products of CO, reduction are methanol and ethanol but this depends upon the electrolyte used For example, when KHCO, was used as an electrolyte, methanol and ethanol were produced almost equally On the other hand, the amount of methanol produced was more than ten times that of ethanol in Na 2SO, solution 3. NEW PHOTOCATALYTIC APPLICATIONS FOR DEODORIZATION Improvement in the surrounding environment which we encounter daily is being recognized as an increasingly important topic in modern society For example, deodonzation and sterilization systems in closed spaces such as an office room, underground market and the inside of vehicles are becoming more important, especially in public areas It is desirable that such systems are free of maintenance and do not perturb the general appearance of their surrounding We have succeeded in developing a new type of deodorizing and sterilizing material based on TiO, which works semipermanently under room light illumination and without the need for any maintenance Photoeatalytic TIO 2 thin film is colorless and transparent and can be easily coated onto various building materials Moreover, no additional light sources nor mechanical system is needed for its operation The small amount of ultraviolet (uv) light contained in a regular fluorescent light or halogen lamp is sufficient for the photocatalytic decomposition of odonng gases with this TiO2 thin film Photocatalytic reaction with TiO, proceeds by bandgap (3 eV) excitation using uv light (wavelengths shorter than 400 nm) Many studies have been done in connection with solar energy conversion, carbon dioxide reduction, organic synthesis, water punfication and decomposition of crude oil spill However in these studies, strong solar light or artificial uv light such as a high pressure mercury lamp which contains strong uv light, are used as an excitation light source Moreover, most of these applications use small particle T9O2 powder itself or powder supported on large surface area materials, such as ceramic honeycomb, silica or glass beads, as photocatalysts Here we propose a new type of photocatalytic reaction based on a completely different idea Room light, such as a regular fluorescent light,
Effects and applications of thin semiconductor films and colorless transparent TtO2 films coated on various substrates, such as ceramic tile, are used as an excitation light source and photocatalyst, respectively Ths photocatalytic reaction system does not need any extra system and therefore can be easily introduced to the living space TiO= sot (TiO= content 4-6%, pH 11) was coated at room temperature onto commercially available ceramic tile (Pyrex glass plate) and a 99 9% alumina substrate by spray coating, dip coating or spin The size of the substrate was coating 100mm x 100mm The substrate was then heated at 650°C in air for 1 h The resulting film was colorless and transparent The thickness of the TiO= was about 03-05µm, but interference color was scarcely observed The crystal phase was identified as anatase by X-ray diffraction and Raman scattering measurements A scanning electron micrograph showed that the film was relatively porous and consisted of small sintered particles with an average size of several tens of nanometers The film strongly adhered to the substrates and could not be removed from the ceramic tile by scrubbing For the deodonzation experiments, the TiO 2 coated substrate was set in a glass vessel with a volume of 1-121 Various bad smelling gases such as methyl mercaptan, hydrogen sulfide and acetaldehyde were introduced into the vessel with a concentration ranging from a few ppm to thousands of ppm It was found that malodor gases with ppm order can be decomposed efficiently even under room light illumination (see Fig 1) For the sterilization experiments, about 20,000 bacteria (methicillinresistant bacteria) (MRSA) or Eschencha cob were deposited on TiO, coated and non-coated tiles over an area of 20 cm The effect of irradiation on the sterilization was surprisingly large with most of the bacteria killed even under room light illumination on the TiO2 coated tile, as plotted in Fig . 2
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methods[6, 7] Electroless plating is currently being utilized for the fabrication of printed circuit boards and other coating technology In the conventional electroless plating method (see Fig . 3a), the substrate surface is initially etched to increase the adhesion between the substrate and obtained metal layer . Next, the substrate surface is activated by treating the sample in a tin chloride solution which is then followed by immersing the surface in a palladium chloride solution The palladium colloidal particles are formed onto the surface and act as a catalyst for electroless plating. Finally a metal layer is deposited all over the surfaces by dipping the catalyzed substrate into an electroless plating bath In general, a metal layer obtained on a glass substrate via electroless plating method has a weaker adhesion strength than that prepared by vacuum deposition Recently we developed a new electroless plating method in which a metal layer can adhere strongly to an unetched glass substrate This method uses a ZnO thin film between the glass substrate and metal layer A brief description of our procedure is as follows (see Fig 3b) 1 Preparation and annealing of ZnO thin film
A ZnO thin film was prepared by spray pyrolysis method partly because this method is one of the easiest and least expensive[8] Glass, soda glass and Corning 7059 glass, having a surface roughness (Ra) of less than 0 01 pin, were used as substrates The glass substrates were initially degreased in ethanol solution and rinsed with detonized water A solution of 0 05 M zinc acetate in ethanol was sprayed onto the substrates A glass atomizer was used to obtain very uniformly sized, minute droplets This was achieved by placing the spray head perpendicular to the substrate at a distance of 35cm Nitrogen was used as a carrier gas and the spray rate was adjusted
4. METALIZATION BY (PHOTO)CHEMICAL Substrate REACTIONS OF SEMICONDUCTOR THIN FILMS Etching
I I
Substrate
Spray pyrolysis Annealing i and In the electronic and optical industries, glass subI ~- ZnO film strates are commonly utilized because of their transparent characteristic The metalization of these glass Activation substrates is a necessary step for fabricating elec(SnClz solution) Catalyzation tromc and optical devices At present, vacuum (PdC 2 solution) evaporation, sputtering and electron beam methods (vwvvrvr~-Sn(11) are mainly used to deposit a metal layer on glass 1 1 However it is difficult to obtain strong adhesion I t Catalyzauon strength directly between a metal layer and glass (PdC12 solution) substrate[4] In order to increase the adhesion Electroless copper plating strength a thin film of Cr which plays the role of an tvfvvvv`vvF_ (o) "adhesive" between metal layer and glass substrate is cu widely used[5] For example, in conventional manuElectroless copper facturing of photomasks or circuit boards, Cr and plating Au or Cu were sequentially deposited onto glass (b) substrates by a vacuum deposition method i ,, Co However, vacuum deposition systems are not effec ~tive for fabrication speed and coat . On the other hand, electroless plating a ideally (a) swted for metalization on a mass production scale Fig 3 Schematic diagram of clectroless copper plating because it uses simple chemical reactions instead of processes using (a) conventional method and (b) new the more expensive and time consuming vacuum method
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to 5mlnun - ' Substrate temperature was maintained at 400°C to decompose the zinc acetate to zinc oxide during film growth The deposited ZnO thin film was cooled to room temperature and then annealed at 400°C in air to relieve the stress The growth rate of ZnO thin film was about 001 pmmm"' Scanning electron microscope observation of a 0 8 pm thick ZnO thin film revealed very small particles of less than 0 1 pin diameter X-ray diffractometer measurements indicated that the ZnO film was polycrystalline with crystallites preferentially oriented in the (002) direction The films were highly transparent with a transmittance greater than 80% in the visible region 2 (immersion in PdCl 2 acidic solution
ZnO/glass substrate was activated for electroless copper deposition by immersing in a PdCl, acidic solution The solution was prepared by dissolving 0 2 g PdCI Z in 0 5 ml conc HCI and diluting the solution to I 1 with detonized water The initial pH of the solution was carefully adjusted with diluted HCI or NH 3 (aq) In this process, two phenomena were observed to occur simultaneously One was the change in the color of the ZnO thin film After the ZnO/glass substrate was dipped into PdCI Z acidic solution (pH 2 5) for 2 min, the transparent surface of the ZnO thin film turned yellow This phenomenon occurred only on the ZnO surface On the other hand, in the conventional electroless method, the substrate surface turns gray or black after treatment in SnCI2 and PdCl, acidic solutions because palladium particles were deposited on the substrate surface[9] Thus the valent state of palladium is apparently different in our process, and the palladium seems to be selectively adsorbed as ions onto the ZnO surface Another phenomenon observed was the dissolution of ZnO The dissolution rate depended on the pH of the PdC1 2 solution For example, a 02mm thick ZnO film was completely removed from the substrate within 30s in a PdCI Z
Fig 4
et al
acidic solution at pH 10 and within 10 mm at pH 2 5 However, the dissolution of the ZnO surface was not observed for pH greater than 50 because the ZnO thin film is stable above pH 4 5[10, 11] Moreover, the change of color onto ZnO thin films does not occur for pH above 50 These results indicate that the activation reaction depends upon the dissolution of ZnO 3 Immersion into the electroless copper plating bath The electroless copper plating was carried out using a commercial bath (Ebana-Udybte, Co, Ltd PB-503) for 10min at room temperature and then immersed into another commercial bath (EbaraUdylite, Co, Ltd PB-542) for 60 mm at 60°C while stirring and bubbling the solution The electroless copper plating reaction proceeded easily when the activated substrate was immersed into the electroless copper plating bath (PB-503) However, the deposition occurred selectively on the ZnO thin film rather than over the entire substrate surface as in the case of conventional methods Furthermore, the color of the ZnO surface changed from yellow to black, after which the copper particles were deposited onto the ZnO thin film It appears that a palladium catalyst is needed before the electroless plating reaction can proceed and, therefore, the selectively formed activated palladium on the ZnO thin film is converted Figure 4 shows a SEM micrograph cross section of a film produced using this new plating method The copper film had a mirror-like finish, as shown in Fig 4, because the glass substrate was not etched Even without the etching process, the copper film adhered very strongly The adhesion strength of the copper film was 10 8 kg per 2 mm x 2 mm square for a 3 µm thick film The obtained value is similar to the adhesion strength found for as-deposited Cr and Cu sequentially on a glass substrate by sputtering and enough for practical use This result indicates that the ZnO thin film serves as a kind of "adhesive" in electroless copper plating, similar to the role which a Cr film plays for vacuum deposited films
Scanning electron micrograph showing the side vie of copper and ZnO hin film on Corning 7059
Effects and applications of thin semiconductor films 5. VISIBLE LIGHT INDUCED PHOTOCHROMISM Photochromic materials, whose optical absorption properties change in response to light, are important for a number of technological applications such as optical displays and high-density memory devices We have found that visible-light sensitive photochromism can occur in vacuum-deposited MoO3 thin films after electrolytic pretreatment[12-17] The coloration could easily be erased by anodic polarization and the coloration-decoloration process repeated over many cycles The MOO, amorphous film was prepared by vacuum evaporation of highly pure MOO, powders onto a 1 mm thick NESA glass substrate Photochromic experiments were done in air, and a 50O W high pressure mercury lamp was used as a light source For bandgap excitation, the light fell directly on the sample, whereas for the visible-light experiments, a sharp cut-off filter was used to obtain light of wavelength >500nm . In the electrochromic experiments, the sample was dipped into a O 1 M LiCIO4/propylene carbonate solution Platinum and Ag/AgCI electrodes were used as counter and reference electrodes, respectively Since freshly prepared MOO, is colorless and transparent, it does not show photochromism or irradiation by visible light However, after being slightly blued by electrochromism, it becomes sensitive to visible light Figure 5a shows the changes in the absorption spectrum of the MOO, thin film, with spectra for both the prepared amorphous film and the electrolytically prepared film These results were obtained as follows First, the absorption spectrum of the prepared MOO, colorless film was recorded (curve A) Next, the film was polarized at -0 1 V vs Ag/AgCI for 3 mm in the electrolyte The film was slightly colored, as indicated by curve B The change in absorbance from curve A to curve B was -0 05 at 800nm This electrolytically prepared film was then
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irradiated with visible light for 10mm in air The irradiated part turned deep blue (curve C) with the change in absorbance being -03 at 800 min Similar color enhancement was also observed using nearinfrared irradiation (A > 700nm) This result indicates that even when excited by a broad absorption band (500-1000nm), the photochromic reaction is induced in this pretreated MoO S film Figure 5b shows, in contrast, that a MoO 3 film blued by bandgap irradiation pretreatment is not sensitive to visible light irradiation in air, even though the absorption spectrum of a film blued in this way (curve E) is very similar in shape to that of a film blued by the electrochromic reaction After coloration by visible light irradiation, the MOO, thin film can be exposed in air for a prolonged period No absorbance change was detected for films continuously exposed in all seven months after the initial coloration, indicating that the colored film was very stable in air The MOO, film colored by visible light irradiation also showed good reversibility in its coloration-decoloration process If the film was completely decolored, it was no longer sensitive to visible light But if the colorless film was slightly blued again, it became sensitive to visible light Conventional photochromic processes are explained as follows When a thin film of MOO, is irradiated by ultraviolet light, electrons and holes are formed, thereby allowing the photogenerated holes to react with surface absorbed species, causing the film to be negatively charged The positive ions on the surface, protons in the present case, are then injected into the film by Coulomb attraction, forming the hydrogen molybdenum bronze The observed visible light induced photochromic reaction might, however, originate from a different reaction mechanism The broad absorption band in the visible region is due to the charge-transfer transition from Mo 5+ to the oxide ligands, producing Mo` Therefore, it is apparent that the excitation of this
Main) Fig. 5 Absomon spectra of vacuum-deposited MOO, film (curve A) as prepared, (curve 0) after polanzatam at -01 V vs. Ag/AgCI for 3vm in 01 M LtCIO,Jpropylene carbonate solution, (curve C) irradiated with visible light (> 500mm) for 10 mm m air after measuring curve B, (curve D) as prepared, (curve E) irradiated with ultraviolet light for 8nun in air, (curve F) irradiated for 10mm in air after measuring curve E
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(write) visible irradiation Anodic polarization 1 - (Erase) Stage C
Stage B
Fig 6 Proposed application for a display device using the visible light enhanced photochromic behavior of MOO, broad band does not produce electron-hole pairs in the film We suggest that the MoO 3 film changes to an intermediate metastable state during the electrolytic reaction and is then converted by visible light into stable molybdenum bronze Figure 6 shows a scheme using a combination of the "electrochemical mode" and the "photon mode" in a MoO 3 thin film to achieve a novel display device For example, starting with a colorless MoO 3 film (stage A), it is slightly blued by cathodic polarization (electrochemical mode) to generate a visible light sensitive material (stage B) Then the desired pattern is written on it by visible light irradiation (photon mode, stage C) This pattern is easily erased either to the pretreated stage (stage B) or to the initial stage (stage A) by controlling the time for anodic polarization, afterwhich the same process can be used again repeatedly
6. RAMAN MAPPING OF MoO 3 THIN FILMS Raman imaging analysis[18, 19], has attracted renewed interest recently, partly due to several advances in technology such as line-scanned laser beams[20] and the acousto-optic tunable filter[21] The combination of holographic laser rejection filters, high throughput monochrometers with band pass filters, and highly sensitive CCD cameras have made it possible to acquire Raman images of semiconductors in several minutes[22] Here, we show that due to its ease of operation and high spatial resolution (1 pm), Raman imaging analysis can be used in the study of crystallization of thin MoO, oxide film at the micrometer scale The Raman signals of crystallized MOO, are much higher than the signal for amorphous MOO„ and thus it is relatively easy to distinguish between the two types of regions The MOO, thin films were prepared by vacuum evaporation of highly pure MoO 3 powder onto mm thick ITO glass substrates at a pressure of 5 x 10 -5 Torr The thickness of the deposited films was about lµm[15] Crystallization of freshly prepared MOO,
films by laser annealing was difficult due to their transparency To assist the crystallization process, the film was colored blue by electrochemically reducing the Moa' to Me" in a solution of 0 1 M LiCIO 4 and propylene carbonate solution at a constant current density of 30pAcm -2 for 60mm[23, 24] The crystallization of the MOO, films was performed using an argon ion laser (5145nm) at an intensity of 1 5 mW µm -2 The laser was focused to an area with a diameter of approximately 2 µm using the 50 x objective lens of a microprobe Raman spectrometer (Renishaw, Ramascope) Crystalline MOO, lines were fabricated by rastering the laser at a high speed (-'2pms - ') To obtain the Raman images, a HeNe laser (632 8 not), set at 9 pW µm"', was used Images were obtained using a spot with a diameter of approximately 70µm A Raman image was taken using a multilayer dielectric filter centered at 818 cm - ' with a 20 cm - ' bandwidth, which is the main Raman shift of the MoO, crystal This shift corresponds to the Mo-O-Mo stretching mode As mentioned above, laser anealing of MOO, films prior to electrochemical reduction produced no noticeable Raman peaks characteristic of crystallized MoO 3 films prior to electrochemical reduction produced no noticeable Raman peaks characteristic of crystallized MOO, films In contrast, distinct Raman peaks (solid line, Fig 7) are clearly seen on samples that were electrochromically colored blue and subsequently laser annealed high intensity at (1 5 mW pm - ') These Raman bands are assigned to the terminal stretching mode at 995cm-' (as, b i ,), the two-connected bridge mode at 818c_ i (aa , b ia), and the three-connected bridge stretching modes at 470 cni - ' la, b ia) and 665cm - ' (b2a, b373 in accord with MOO, powder crystal Raman bands[25] However, the half-width, half-maximum (HWHM) (see Fig 7) at 818cm - ' of the crystallized MoO, film is much bigger than that found in MOO, powder crystals The Raman spectrum of a colored thin MoO 3 film annealed at lower laser power (08mWpm - ') is also shown as the dashed line in Fig 7 No distinguishable peaks are observed even
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Raman shift/cm Fig 7 Raman spectra of a colored amorphous thin (-'1µm) film of MoO 3 prepared by cathodic polarization at a constant current of 30pAcm - ' for 60mm on vacuum evaporation before (dash line) and after laser annealing (solid line)
Effects and applications of thin semiconductor films after irradiating the film for 10min, which indicates that the film was still amorphous Since both sets of bands are wide, both the crystallized and amorphous regions are believed to be highly defective Figure 8a shows an optical micrograph (approx 85 pin x 85 µm) of a MoO3 film after laser annealing The lines are separated by approximately 15 pm The MoO3 film prepared by vacuum evaporation was very smooth, however, small cracks (02µm)
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appeared after immersion in the O 1 M LiC9O,/propylene carbonate solution The width of cracks became wider (> 1µm) and the surface rougher due to the stress generated during laser annealing A Raman image taken using a multilayer dielectric bypass filter centered at S18cm - ' with a 20cm - ' band width, is shown in Fig 8(b) The image was taken with a 300s exposure time The brightness of
(a)
(b)
20 pm ( i Fig 8 (a) Optical micrograph of a MoO 3 film after laser annealing The lines shown are separated by approx 15µm (b) A Raman image taken at 818= - ' (Mo-O-Mo stretching mode) The darker regions represent amorphous MOO S and brighter regions represent crystallized MoO 3
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A FunsHIMA et at
the image indicates the degree of crystallization, with undoubtably bring more exciting discoveries in both the darker regions representing a more amorphous basic and applied research for many years to come MoO 3 structure and brighter regions representing the more crystallized MoO 3 The lines of crystallized MoO 3 after laser annealing in this Raman image are much clearer than can be seen in an optical microREFERENCES graph This Raman image was obtained by subtracting a background image centered at 738 cm - ' from I A Fujishima and K Honda, Nature 238, 37 (1972) that taken at 818 cm -1 in order to remove the broad 2 A Fujishima and K Honda, Bull Chem Soc Japan 44, band luminescence of any material (eg 1148 (1971) contaminants) As a result of this procedure, 3 Y Hori, K Kikuchi and S Susuki, Chem Lett 1965 amorphous Raman bands, which have almost the (1985) same intensities at 81Scm - ' and 738cm"', are also 4 P Benjamin and C Weaver, Proc Roy Soc A 261, 516 removed Since the laser spot was uniform only near (1961) 5 N M Poley and H L Whitaker, J Vac So Technol the center, the annealed lines at the top and bottom 11, 114 (1974) cannot be seen clearly This work shows the relative 6 S Wan, Glass Ind 38, 625 (1957) ease of obtaining high spatial resolution (1µm), 7 C R Shipley Jr, U S Patent, 3,011,920 (1961) Raman imaging analysis on thin oxide films and can 8 T Osaka, H Takematsu and K Nihei, J electrochem be used in the investigation of crystallization in other Soc 127,1021(1980) semiconductor materials 9 J Domenech and A Prieto, J phys Chem 90, 1123 (1986) 10 T Osaka, H Nagata, E Nakalima and I Koiwa, J 7. CONCLUSION electrochem Soc 133,2345 (1986) 11 T Osaka, T Asada, E Nakajima and I Koiwa, J electrochem Soc 135, 2578 (1988) In summary, we have reported various applica12 J N Yao, B H Loo and A Fupshima, Ber Bunsenges tions of thin semiconductor films under the influence Phys Chem 94, 13 (1990) of light We have shown that photoelectrochemical 13 J N Yao, B H Loo, K Hashimoto and A Fupshima, reduction of CO, on p-type copper oxide and silicon J electronal Chem 290, 263 (1990) carbide electrodes can be used to produce useful pro14 J N Yao, B H Loo, K Hashimoto and A Fujishima, ducts such as methanol Thin TiO, films exposed to Ber Bunsenges Phys Chem 95, 554(1991) regular fluorescent light can be used in the removal 15 J N Yao, B H Loo, K Hashimoto and A Fupshima, of malodor gases TiO, appears to be an ideal Ber Bunsenges Phys Chem 95, 557 (1991) 16 J N Yao, K Hashimoto and A Fujishima, Nature material for such a passive system since it is colorless 355,624 (1992) and adheres very strongly to most building 17 J N Yao, B H Loo, K Hashimoto and A Fujishima, materials We have also demonstrated that a thin Ber Bunsenges Phys Chem 96, 669 (1992) layer of ZnO acts as an effective adhesion layer 18 M Delhaye and P Dhamelmcourt, J Raman Spectrosc between a metal layer and underlying glass sub3,33(1975) strate Strong adhesion strength between a metal 19 P Dhamelmcourt, F Wallart, M Leclcrcq, A T layer and glass substrate will be needed in order to N'Guyen and D 0 Landon, Anal Chem 51, 414 improve the performance of future electronic and (1979) optical devices In addition, we have shown that 20 M Bowden . D J Gardiner, G Rice and D L Gerrard, J Raman Spectrosc 46, 1211(1992) photochromism can be induced with visible light in 21 P 1 Treado, I W Levin and E him l Lewis, Appl SpecMoO3 after pretreatment in an electrolytic solution trosc 21, 37 (1990) Moreover, we have shown that Raman imaging can 22 R V Sudiwala, C Cheng, E G Wilson and D N be used to distinguish quite easily at micrometer Batchelder, Thin Solid Films 210/211, 452 (1992) resolution the difference between the amorphous and 23 1 W Rabalasis, R J Colton and A M Guzman, crystalline phase of MoO 3 Similar studies on Chem Phys Lett 29, 131 (1974) various other semiconductor materials are possible 24 T C Arnoldusscn, J electrochem Sac 123, 527 (1976) Finally, continued investigation in the area of light 25 1 R Beattie and T R Gibson, J Chem Soc (A) 2322 interaction wth semiconductor electrodes will (1969)
Pergamon
Actq Vol 39.N. 8/9, pp IM-1236.1994 Copyright C 1994 Elrr,et Samuel Ltd Punted to Oral annum All nghn nuwed 0013-4686191$700 +nr
0013.4686(94)E0041-W
THIN SEMICONDUCTOR FILMS : PHOTOEFFECTS AND NEW APPLICATIONS FunsmxsA, • LARRY A NAGAHAIA, HAnhtE Yosmta, KA7suimeo Ante and K wir o HAsimtow Department of Synthetic Chemistry, Faculty of Engmeermg, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan AURA
(Received 7 September 1993) Abstract--Current topics in photochemistry and photoelectrochemlstry of thin semiconductor films are presented in regards to technological applications Examples include photoelectrochenical reduction of CO s , photocatalytic applications for deodorizahon, metalization via photochemical reactions, and the photochromic behavior of MoO 3 Key words semiconductor photoelectrochemistry, CO, reduction, photocatalyst, photochemical metalization, photochromism, Raman imaging 1. INTRODUCTION The interaction of light with semiconductor elect trodes has been studied for more than forty years, but it was not until the discovery of water photoelectrolysis at a TiO, electrode[1, 2] and subsequent the worldwide oil shortage in the 1970s that extensive research on photoeffects and semiconductor electrode research involve practical applications . During this period, solar energy conversion received the most attention as a way to solve the energy crisis However since that time, many other novel applications involving the use of photon energy with semiconductor electrodes have been reported For example, the need to solve certain environmental issues such as carbon dioxide reduction, deodorization/stenlization of public spaces and water purification have received much attention The main purpose of this paper is to present some recent applications of utilizing semiconductor thin films under the influence of light Specifically, we will discuss five areas of application (1) photoeleotrochemical reduction of CO, using p-type semiconductor electrodes, (2) photocatalytic materials for deodonzation and sterilization, (3) metalizahon on thin semiconductor layers, (4) visible-light induced photochromism in MoO3 , and (5) determination of the structural phase in photochromic materials using Raman imaging 2. SEMICONDUCTOR ELECTRODES FOR CO, REDUCTION Electrochemical carbon dioxide reduction has attracted renewed interest among researchers recently The reaction has been extensively studied over the years, however this renewed interest in this • Keynote lecture presented at the 44th ISE Meeting in Berlin
field originates from the following two reasons One is related to the so called "greenhouse effect" The other is the discovery by Hon et at [3] that carbon dioxide can be reduced to methane using a Cu electrode, with a current efficiency of almost 100% . When the electrochemical carbon dioxide reduction method is studied in relation to the "greenhouse effect", the problem of energy source should be taken into consideration Here, the electrochemical carbon dioxide reduction using light energy as the first energy source is presented The electrochemical systems using light energy can be classified into two categories . One is the convertbona] electrochemical system using a metal electrode equipped with a dry solar battery such as amorphous St In this system, solar energy is first converted to electric energy, then the electric energy is converted to chemical energy The other category is the photoelectrochemical system in which light energy is directly converted to chemical energy from a semiconductor electrode There have been many reports on the photoelectrochemrcal carbon dioxide reduction using various semiconductor electrodes such as GaP and ZnTe The main products reported thus far, however, are CO or HCOOH in aqueous solution Our intent is to obtain more valuable products by using photoelectrochenucal methods under milder cathodic conditions We have successfully applied two new p-type semiconductors as cathodes, namely copper oxide and silicon carbide, which selectively produce methanol from carbon dioxide and water We believe that this is the first reported case of electrochemically forming highly reduced products such as CH,, (eight electrons) and CH 3OH (six electrons) as main products with the aid of photon energy Copper oxide electrode The CuO electrode was prepared by the electrochemical oxidation of a pure Cu metal electrode . First, the Cu electrode was oxidized at a potential of
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Fig I Plots of the decomposition of acetaldehyde as a function of time for (a) high concentration and (b) low concentration Even at a concentration of a few prim, decomposition proceeds very efficiently under fluorescent light BL = black fluorescent light W L = white fluorescent light +0 3 V vs sce until 1 C of charge had passed At this
potential, Cu was oxidized to CuO Next, the CuO electrode was cathodically biased between a potential range of -01 to -04V vs sce in CO, saturated 0 1 M KHCO 3 aqueous solution A 500 W Xc lamp was used as a light source The main reduction product at this potential was CH, However, the CuO electrode was also reduced to Cu,O and then subsequently converted into Cu in this potential range Therefore, the reduction of CO, competes with the reduction of the CuO electrode itself and for this reason, the rate of CH, formation decreases gradually The highest current efficiency of CH, was initially obtained with about 60% at -0 3 V vs sce At this stage, no other reduction products were observed However, after prolonged electrolysis,
a White fluorescent light -~- Black light
0
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Irradiation time/h Fig 2 Plot of the fraction of MRSA bacteria surviving on a TI0 2 coated the vs time MRSA = methic lhn-resistant staphylococcus aureus
CH,OH was also detected with current efficiencies reaching 20% This presumably occurred due to the increased formation of Cu 2O on the electrode SIC electrode Single crystal and a microcrystalline p-type SIC were used as the electrodes in the present study In the case of single crystal SIC, the photocurrent was very small and hydrogen production dominated the reaction Hence we could not detect any reduction products of CO 2 In contrast, a microcrystalline SIC film electrode, which was deposited on indium tin oxide (ITO) by photoassisted (mercury lamp light) chemical vapor deposition (CVD) gave a considerable amount of reduction products of CO2 Although H, was still produced with a current efficiency of 80%, the rest of the electrons were used for the reduction of CO 2 Moreover, it should be stressed that the main products of CO, reduction are methanol and ethanol but this depends upon the electrolyte used For example, when KHCO, was used as an electrolyte, methanol and ethanol were produced almost equally On the other hand, the amount of methanol produced was more than ten times that of ethanol in Na 2SO, solution 3. NEW PHOTOCATALYTIC APPLICATIONS FOR DEODORIZATION Improvement in the surrounding environment which we encounter daily is being recognized as an increasingly important topic in modern society For example, deodonzation and sterilization systems in closed spaces such as an office room, underground market and the inside of vehicles are becoming more important, especially in public areas It is desirable that such systems are free of maintenance and do not perturb the general appearance of their surrounding We have succeeded in developing a new type of deodorizing and sterilizing material based on TiO, which works semipermanently under room light illumination and without the need for any maintenance Photoeatalytic TIO 2 thin film is colorless and transparent and can be easily coated onto various building materials Moreover, no additional light sources nor mechanical system is needed for its operation The small amount of ultraviolet (uv) light contained in a regular fluorescent light or halogen lamp is sufficient for the photocatalytic decomposition of odonng gases with this TiO2 thin film Photocatalytic reaction with TiO, proceeds by bandgap (3 eV) excitation using uv light (wavelengths shorter than 400 nm) Many studies have been done in connection with solar energy conversion, carbon dioxide reduction, organic synthesis, water punfication and decomposition of crude oil spill However in these studies, strong solar light or artificial uv light such as a high pressure mercury lamp which contains strong uv light, are used as an excitation light source Moreover, most of these applications use small particle T9O2 powder itself or powder supported on large surface area materials, such as ceramic honeycomb, silica or glass beads, as photocatalysts Here we propose a new type of photocatalytic reaction based on a completely different idea Room light, such as a regular fluorescent light,
Effects and applications of thin semiconductor films and colorless transparent TtO2 films coated on various substrates, such as ceramic tile, are used as an excitation light source and photocatalyst, respectively Ths photocatalytic reaction system does not need any extra system and therefore can be easily introduced to the living space TiO= sot (TiO= content 4-6%, pH 11) was coated at room temperature onto commercially available ceramic tile (Pyrex glass plate) and a 99 9% alumina substrate by spray coating, dip coating or spin The size of the substrate was coating 100mm x 100mm The substrate was then heated at 650°C in air for 1 h The resulting film was colorless and transparent The thickness of the TiO= was about 03-05µm, but interference color was scarcely observed The crystal phase was identified as anatase by X-ray diffraction and Raman scattering measurements A scanning electron micrograph showed that the film was relatively porous and consisted of small sintered particles with an average size of several tens of nanometers The film strongly adhered to the substrates and could not be removed from the ceramic tile by scrubbing For the deodonzation experiments, the TiO 2 coated substrate was set in a glass vessel with a volume of 1-121 Various bad smelling gases such as methyl mercaptan, hydrogen sulfide and acetaldehyde were introduced into the vessel with a concentration ranging from a few ppm to thousands of ppm It was found that malodor gases with ppm order can be decomposed efficiently even under room light illumination (see Fig 1) For the sterilization experiments, about 20,000 bacteria (methicillinresistant bacteria) (MRSA) or Eschencha cob were deposited on TiO, coated and non-coated tiles over an area of 20 cm The effect of irradiation on the sterilization was surprisingly large with most of the bacteria killed even under room light illumination on the TiO2 coated tile, as plotted in Fig . 2
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methods[6, 7] Electroless plating is currently being utilized for the fabrication of printed circuit boards and other coating technology In the conventional electroless plating method (see Fig . 3a), the substrate surface is initially etched to increase the adhesion between the substrate and obtained metal layer . Next, the substrate surface is activated by treating the sample in a tin chloride solution which is then followed by immersing the surface in a palladium chloride solution The palladium colloidal particles are formed onto the surface and act as a catalyst for electroless plating. Finally a metal layer is deposited all over the surfaces by dipping the catalyzed substrate into an electroless plating bath In general, a metal layer obtained on a glass substrate via electroless plating method has a weaker adhesion strength than that prepared by vacuum deposition Recently we developed a new electroless plating method in which a metal layer can adhere strongly to an unetched glass substrate This method uses a ZnO thin film between the glass substrate and metal layer A brief description of our procedure is as follows (see Fig 3b) 1 Preparation and annealing of ZnO thin film
A ZnO thin film was prepared by spray pyrolysis method partly because this method is one of the easiest and least expensive[8] Glass, soda glass and Corning 7059 glass, having a surface roughness (Ra) of less than 0 01 pin, were used as substrates The glass substrates were initially degreased in ethanol solution and rinsed with detonized water A solution of 0 05 M zinc acetate in ethanol was sprayed onto the substrates A glass atomizer was used to obtain very uniformly sized, minute droplets This was achieved by placing the spray head perpendicular to the substrate at a distance of 35cm Nitrogen was used as a carrier gas and the spray rate was adjusted
4. METALIZATION BY (PHOTO)CHEMICAL Substrate REACTIONS OF SEMICONDUCTOR THIN FILMS Etching
I I
Substrate
Spray pyrolysis Annealing i and In the electronic and optical industries, glass subI ~- ZnO film strates are commonly utilized because of their transparent characteristic The metalization of these glass Activation substrates is a necessary step for fabricating elec(SnClz solution) Catalyzation tromc and optical devices At present, vacuum (PdC 2 solution) evaporation, sputtering and electron beam methods (vwvvrvr~-Sn(11) are mainly used to deposit a metal layer on glass 1 1 However it is difficult to obtain strong adhesion I t Catalyzauon strength directly between a metal layer and glass (PdC12 solution) substrate[4] In order to increase the adhesion Electroless copper plating strength a thin film of Cr which plays the role of an tvfvvvv`vvF_ (o) "adhesive" between metal layer and glass substrate is cu widely used[5] For example, in conventional manuElectroless copper facturing of photomasks or circuit boards, Cr and plating Au or Cu were sequentially deposited onto glass (b) substrates by a vacuum deposition method i ,, Co However, vacuum deposition systems are not effec ~tive for fabrication speed and coat . On the other hand, electroless plating a ideally (a) swted for metalization on a mass production scale Fig 3 Schematic diagram of clectroless copper plating because it uses simple chemical reactions instead of processes using (a) conventional method and (b) new the more expensive and time consuming vacuum method
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to 5mlnun - ' Substrate temperature was maintained at 400°C to decompose the zinc acetate to zinc oxide during film growth The deposited ZnO thin film was cooled to room temperature and then annealed at 400°C in air to relieve the stress The growth rate of ZnO thin film was about 001 pmmm"' Scanning electron microscope observation of a 0 8 pm thick ZnO thin film revealed very small particles of less than 0 1 pin diameter X-ray diffractometer measurements indicated that the ZnO film was polycrystalline with crystallites preferentially oriented in the (002) direction The films were highly transparent with a transmittance greater than 80% in the visible region 2 (immersion in PdCl 2 acidic solution
ZnO/glass substrate was activated for electroless copper deposition by immersing in a PdCl, acidic solution The solution was prepared by dissolving 0 2 g PdCI Z in 0 5 ml conc HCI and diluting the solution to I 1 with detonized water The initial pH of the solution was carefully adjusted with diluted HCI or NH 3 (aq) In this process, two phenomena were observed to occur simultaneously One was the change in the color of the ZnO thin film After the ZnO/glass substrate was dipped into PdCI Z acidic solution (pH 2 5) for 2 min, the transparent surface of the ZnO thin film turned yellow This phenomenon occurred only on the ZnO surface On the other hand, in the conventional electroless method, the substrate surface turns gray or black after treatment in SnCI2 and PdCl, acidic solutions because palladium particles were deposited on the substrate surface[9] Thus the valent state of palladium is apparently different in our process, and the palladium seems to be selectively adsorbed as ions onto the ZnO surface Another phenomenon observed was the dissolution of ZnO The dissolution rate depended on the pH of the PdC1 2 solution For example, a 02mm thick ZnO film was completely removed from the substrate within 30s in a PdCI Z
Fig 4
et al
acidic solution at pH 10 and within 10 mm at pH 2 5 However, the dissolution of the ZnO surface was not observed for pH greater than 50 because the ZnO thin film is stable above pH 4 5[10, 11] Moreover, the change of color onto ZnO thin films does not occur for pH above 50 These results indicate that the activation reaction depends upon the dissolution of ZnO 3 Immersion into the electroless copper plating bath The electroless copper plating was carried out using a commercial bath (Ebana-Udybte, Co, Ltd PB-503) for 10min at room temperature and then immersed into another commercial bath (EbaraUdylite, Co, Ltd PB-542) for 60 mm at 60°C while stirring and bubbling the solution The electroless copper plating reaction proceeded easily when the activated substrate was immersed into the electroless copper plating bath (PB-503) However, the deposition occurred selectively on the ZnO thin film rather than over the entire substrate surface as in the case of conventional methods Furthermore, the color of the ZnO surface changed from yellow to black, after which the copper particles were deposited onto the ZnO thin film It appears that a palladium catalyst is needed before the electroless plating reaction can proceed and, therefore, the selectively formed activated palladium on the ZnO thin film is converted Figure 4 shows a SEM micrograph cross section of a film produced using this new plating method The copper film had a mirror-like finish, as shown in Fig 4, because the glass substrate was not etched Even without the etching process, the copper film adhered very strongly The adhesion strength of the copper film was 10 8 kg per 2 mm x 2 mm square for a 3 µm thick film The obtained value is similar to the adhesion strength found for as-deposited Cr and Cu sequentially on a glass substrate by sputtering and enough for practical use This result indicates that the ZnO thin film serves as a kind of "adhesive" in electroless copper plating, similar to the role which a Cr film plays for vacuum deposited films
Scanning electron micrograph showing the side vie of copper and ZnO hin film on Corning 7059
Effects and applications of thin semiconductor films 5. VISIBLE LIGHT INDUCED PHOTOCHROMISM Photochromic materials, whose optical absorption properties change in response to light, are important for a number of technological applications such as optical displays and high-density memory devices We have found that visible-light sensitive photochromism can occur in vacuum-deposited MoO3 thin films after electrolytic pretreatment[12-17] The coloration could easily be erased by anodic polarization and the coloration-decoloration process repeated over many cycles The MOO, amorphous film was prepared by vacuum evaporation of highly pure MOO, powders onto a 1 mm thick NESA glass substrate Photochromic experiments were done in air, and a 50O W high pressure mercury lamp was used as a light source For bandgap excitation, the light fell directly on the sample, whereas for the visible-light experiments, a sharp cut-off filter was used to obtain light of wavelength >500nm . In the electrochromic experiments, the sample was dipped into a O 1 M LiCIO4/propylene carbonate solution Platinum and Ag/AgCI electrodes were used as counter and reference electrodes, respectively Since freshly prepared MOO, is colorless and transparent, it does not show photochromism or irradiation by visible light However, after being slightly blued by electrochromism, it becomes sensitive to visible light Figure 5a shows the changes in the absorption spectrum of the MOO, thin film, with spectra for both the prepared amorphous film and the electrolytically prepared film These results were obtained as follows First, the absorption spectrum of the prepared MOO, colorless film was recorded (curve A) Next, the film was polarized at -0 1 V vs Ag/AgCI for 3 mm in the electrolyte The film was slightly colored, as indicated by curve B The change in absorbance from curve A to curve B was -0 05 at 800nm This electrolytically prepared film was then
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irradiated with visible light for 10mm in air The irradiated part turned deep blue (curve C) with the change in absorbance being -03 at 800 min Similar color enhancement was also observed using nearinfrared irradiation (A > 700nm) This result indicates that even when excited by a broad absorption band (500-1000nm), the photochromic reaction is induced in this pretreated MoO S film Figure 5b shows, in contrast, that a MoO 3 film blued by bandgap irradiation pretreatment is not sensitive to visible light irradiation in air, even though the absorption spectrum of a film blued in this way (curve E) is very similar in shape to that of a film blued by the electrochromic reaction After coloration by visible light irradiation, the MOO, thin film can be exposed in air for a prolonged period No absorbance change was detected for films continuously exposed in all seven months after the initial coloration, indicating that the colored film was very stable in air The MOO, film colored by visible light irradiation also showed good reversibility in its coloration-decoloration process If the film was completely decolored, it was no longer sensitive to visible light But if the colorless film was slightly blued again, it became sensitive to visible light Conventional photochromic processes are explained as follows When a thin film of MOO, is irradiated by ultraviolet light, electrons and holes are formed, thereby allowing the photogenerated holes to react with surface absorbed species, causing the film to be negatively charged The positive ions on the surface, protons in the present case, are then injected into the film by Coulomb attraction, forming the hydrogen molybdenum bronze The observed visible light induced photochromic reaction might, however, originate from a different reaction mechanism The broad absorption band in the visible region is due to the charge-transfer transition from Mo 5+ to the oxide ligands, producing Mo` Therefore, it is apparent that the excitation of this
Main) Fig. 5 Absomon spectra of vacuum-deposited MOO, film (curve A) as prepared, (curve 0) after polanzatam at -01 V vs. Ag/AgCI for 3vm in 01 M LtCIO,Jpropylene carbonate solution, (curve C) irradiated with visible light (> 500mm) for 10 mm m air after measuring curve B, (curve D) as prepared, (curve E) irradiated with ultraviolet light for 8nun in air, (curve F) irradiated for 10mm in air after measuring curve E
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(write) visible irradiation Anodic polarization 1 - (Erase) Stage C
Stage B
Fig 6 Proposed application for a display device using the visible light enhanced photochromic behavior of MOO, broad band does not produce electron-hole pairs in the film We suggest that the MoO 3 film changes to an intermediate metastable state during the electrolytic reaction and is then converted by visible light into stable molybdenum bronze Figure 6 shows a scheme using a combination of the "electrochemical mode" and the "photon mode" in a MoO 3 thin film to achieve a novel display device For example, starting with a colorless MoO 3 film (stage A), it is slightly blued by cathodic polarization (electrochemical mode) to generate a visible light sensitive material (stage B) Then the desired pattern is written on it by visible light irradiation (photon mode, stage C) This pattern is easily erased either to the pretreated stage (stage B) or to the initial stage (stage A) by controlling the time for anodic polarization, afterwhich the same process can be used again repeatedly
6. RAMAN MAPPING OF MoO 3 THIN FILMS Raman imaging analysis[18, 19], has attracted renewed interest recently, partly due to several advances in technology such as line-scanned laser beams[20] and the acousto-optic tunable filter[21] The combination of holographic laser rejection filters, high throughput monochrometers with band pass filters, and highly sensitive CCD cameras have made it possible to acquire Raman images of semiconductors in several minutes[22] Here, we show that due to its ease of operation and high spatial resolution (1 pm), Raman imaging analysis can be used in the study of crystallization of thin MoO, oxide film at the micrometer scale The Raman signals of crystallized MOO, are much higher than the signal for amorphous MOO„ and thus it is relatively easy to distinguish between the two types of regions The MOO, thin films were prepared by vacuum evaporation of highly pure MoO 3 powder onto mm thick ITO glass substrates at a pressure of 5 x 10 -5 Torr The thickness of the deposited films was about lµm[15] Crystallization of freshly prepared MOO,
films by laser annealing was difficult due to their transparency To assist the crystallization process, the film was colored blue by electrochemically reducing the Moa' to Me" in a solution of 0 1 M LiCIO 4 and propylene carbonate solution at a constant current density of 30pAcm -2 for 60mm[23, 24] The crystallization of the MOO, films was performed using an argon ion laser (5145nm) at an intensity of 1 5 mW µm -2 The laser was focused to an area with a diameter of approximately 2 µm using the 50 x objective lens of a microprobe Raman spectrometer (Renishaw, Ramascope) Crystalline MOO, lines were fabricated by rastering the laser at a high speed (-'2pms - ') To obtain the Raman images, a HeNe laser (632 8 not), set at 9 pW µm"', was used Images were obtained using a spot with a diameter of approximately 70µm A Raman image was taken using a multilayer dielectric filter centered at 818 cm - ' with a 20 cm - ' bandwidth, which is the main Raman shift of the MoO, crystal This shift corresponds to the Mo-O-Mo stretching mode As mentioned above, laser anealing of MOO, films prior to electrochemical reduction produced no noticeable Raman peaks characteristic of crystallized MoO 3 films prior to electrochemical reduction produced no noticeable Raman peaks characteristic of crystallized MOO, films In contrast, distinct Raman peaks (solid line, Fig 7) are clearly seen on samples that were electrochromically colored blue and subsequently laser annealed high intensity at (1 5 mW pm - ') These Raman bands are assigned to the terminal stretching mode at 995cm-' (as, b i ,), the two-connected bridge mode at 818c_ i (aa , b ia), and the three-connected bridge stretching modes at 470 cni - ' la, b ia) and 665cm - ' (b2a, b373 in accord with MOO, powder crystal Raman bands[25] However, the half-width, half-maximum (HWHM) (see Fig 7) at 818cm - ' of the crystallized MoO, film is much bigger than that found in MOO, powder crystals The Raman spectrum of a colored thin MoO 3 film annealed at lower laser power (08mWpm - ') is also shown as the dashed line in Fig 7 No distinguishable peaks are observed even
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Raman shift/cm Fig 7 Raman spectra of a colored amorphous thin (-'1µm) film of MoO 3 prepared by cathodic polarization at a constant current of 30pAcm - ' for 60mm on vacuum evaporation before (dash line) and after laser annealing (solid line)
Effects and applications of thin semiconductor films after irradiating the film for 10min, which indicates that the film was still amorphous Since both sets of bands are wide, both the crystallized and amorphous regions are believed to be highly defective Figure 8a shows an optical micrograph (approx 85 pin x 85 µm) of a MoO3 film after laser annealing The lines are separated by approximately 15 pm The MoO3 film prepared by vacuum evaporation was very smooth, however, small cracks (02µm)
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appeared after immersion in the O 1 M LiC9O,/propylene carbonate solution The width of cracks became wider (> 1µm) and the surface rougher due to the stress generated during laser annealing A Raman image taken using a multilayer dielectric bypass filter centered at S18cm - ' with a 20cm - ' band width, is shown in Fig 8(b) The image was taken with a 300s exposure time The brightness of
(a)
(b)
20 pm ( i Fig 8 (a) Optical micrograph of a MoO 3 film after laser annealing The lines shown are separated by approx 15µm (b) A Raman image taken at 818= - ' (Mo-O-Mo stretching mode) The darker regions represent amorphous MOO S and brighter regions represent crystallized MoO 3
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the image indicates the degree of crystallization, with undoubtably bring more exciting discoveries in both the darker regions representing a more amorphous basic and applied research for many years to come MoO 3 structure and brighter regions representing the more crystallized MoO 3 The lines of crystallized MoO 3 after laser annealing in this Raman image are much clearer than can be seen in an optical microREFERENCES graph This Raman image was obtained by subtracting a background image centered at 738 cm - ' from I A Fujishima and K Honda, Nature 238, 37 (1972) that taken at 818 cm -1 in order to remove the broad 2 A Fujishima and K Honda, Bull Chem Soc Japan 44, band luminescence of any material (eg 1148 (1971) contaminants) As a result of this procedure, 3 Y Hori, K Kikuchi and S Susuki, Chem Lett 1965 amorphous Raman bands, which have almost the (1985) same intensities at 81Scm - ' and 738cm"', are also 4 P Benjamin and C Weaver, Proc Roy Soc A 261, 516 removed Since the laser spot was uniform only near (1961) 5 N M Poley and H L Whitaker, J Vac So Technol the center, the annealed lines at the top and bottom 11, 114 (1974) cannot be seen clearly This work shows the relative 6 S Wan, Glass Ind 38, 625 (1957) ease of obtaining high spatial resolution (1µm), 7 C R Shipley Jr, U S Patent, 3,011,920 (1961) Raman imaging analysis on thin oxide films and can 8 T Osaka, H Takematsu and K Nihei, J electrochem be used in the investigation of crystallization in other Soc 127,1021(1980) semiconductor materials 9 J Domenech and A Prieto, J phys Chem 90, 1123 (1986) 10 T Osaka, H Nagata, E Nakalima and I Koiwa, J 7. CONCLUSION electrochem Soc 133,2345 (1986) 11 T Osaka, T Asada, E Nakajima and I Koiwa, J electrochem Soc 135, 2578 (1988) In summary, we have reported various applica12 J N Yao, B H Loo and A Fupshima, Ber Bunsenges tions of thin semiconductor films under the influence Phys Chem 94, 13 (1990) of light We have shown that photoelectrochemical 13 J N Yao, B H Loo, K Hashimoto and A Fupshima, reduction of CO, on p-type copper oxide and silicon J electronal Chem 290, 263 (1990) carbide electrodes can be used to produce useful pro14 J N Yao, B H Loo, K Hashimoto and A Fujishima, ducts such as methanol Thin TiO, films exposed to Ber Bunsenges Phys Chem 95, 554(1991) regular fluorescent light can be used in the removal 15 J N Yao, B H Loo, K Hashimoto and A Fupshima, of malodor gases TiO, appears to be an ideal Ber Bunsenges Phys Chem 95, 557 (1991) 16 J N Yao, K Hashimoto and A Fujishima, Nature material for such a passive system since it is colorless 355,624 (1992) and adheres very strongly to most building 17 J N Yao, B H Loo, K Hashimoto and A Fujishima, materials We have also demonstrated that a thin Ber Bunsenges Phys Chem 96, 669 (1992) layer of ZnO acts as an effective adhesion layer 18 M Delhaye and P Dhamelmcourt, J Raman Spectrosc between a metal layer and underlying glass sub3,33(1975) strate Strong adhesion strength between a metal 19 P Dhamelmcourt, F Wallart, M Leclcrcq, A T layer and glass substrate will be needed in order to N'Guyen and D 0 Landon, Anal Chem 51, 414 improve the performance of future electronic and (1979) optical devices In addition, we have shown that 20 M Bowden . D J Gardiner, G Rice and D L Gerrard, J Raman Spectrosc 46, 1211(1992) photochromism can be induced with visible light in 21 P 1 Treado, I W Levin and E him l Lewis, Appl SpecMoO3 after pretreatment in an electrolytic solution trosc 21, 37 (1990) Moreover, we have shown that Raman imaging can 22 R V Sudiwala, C Cheng, E G Wilson and D N be used to distinguish quite easily at micrometer Batchelder, Thin Solid Films 210/211, 452 (1992) resolution the difference between the amorphous and 23 1 W Rabalasis, R J Colton and A M Guzman, crystalline phase of MoO 3 Similar studies on Chem Phys Lett 29, 131 (1974) various other semiconductor materials are possible 24 T C Arnoldusscn, J electrochem Sac 123, 527 (1976) Finally, continued investigation in the area of light 25 1 R Beattie and T R Gibson, J Chem Soc (A) 2322 interaction wth semiconductor electrodes will (1969)