Bismuth Sulfide Thin Films with Low Resistivity on Self-Assembled

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24054

J. Phys. Chem. B 2006, 110, 24054-24061

Bismuth Sulfide Thin Films with Low Resistivity on Self-Assembled Monolayers Sheng-Cong Liufu, Li-Dong Chen,* Qin Yao, and Chun-Fen Wang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 20050, P R China ReceiVed: August 15, 2006; In Final Form: September 27, 2006

Using self-assembled monolayers (SAMs), highly crystalline bismuth sulfide thin films with low electrical resistivity have been prepared from aqueous solution at low temperature (40-70 °C). The nucleation and growth process of Bi2S3 thin films was investigated in detail by XPS, AES, SEM, XRD, SAED, and HRTEM. Solution conditions have marked effects on the microstructure, growth rate, and mechanism of Bi2S3 films. Increased solution temperature resulted in a higher growth rate and a shorter induction time due to a higher supersaturation degree. In the solution of pH 1.12, homogeneous nucleation and the attachment process dominated the formation of Bi2S3 films. In contrast, at pH 0.47 Bi2S3 thin films were formed via heterogeneous nucleation and growth. The c-axial orientation of bismuthinite films was enhanced with the increase of reaction time. By controlling the solution supersaturation and reaction duration, highly crystalline Bi2S3 films composed of closely packed and coalescent crystallites could be realized, whose dark electrical resistivity could reach as low as 0.014 Ω cm without any post-treatment.

Introduction Bismuth sulfide (Bi2S3) in thin-film form has attracted considerable interest because of not only its applications in photodiode arrays and photovoltaic converters1 but also its potential utilizations in thermoelectric cooling technologies based on the Peltier effect.2 These applications generally require Bi2S3 thin films with low electrical resistivity, which depends greatly on their crystallinity and microstructures. Although highly crystalline and uniform Bi2S3 thin films with relatively low resistivity (about 1 Ωcm)3 are usually obtained by vaporbased techniques [including chemical vapor deposition (CVD)4 and spray pyrolysis (SP)5], they require an expensive investment for special equipment, or special starting materials. Chemical solution methods, such as chemical bath deposition (CBD),1a,6 electrochemical deposition (ECD),7 and successive ion layer adsorption and reaction (SILAR)8 can overcome some defect of vapor-based techniques. But typically the products are poorly crystallized or contain some impurities leading to higher electrical resistivity (102-106 Ωcm),6b,8,9 and post-thermal treatment is generally demanded for decreasing the electrical resistivity.9c,10 In the present study, we develop a simple and mild method for fabricating highly crystalline Bi2S3 thin films. Using self-assembled monolayers (SAMs), Bi2S3 films with low electrical resistivity can be obtained directly without any posttreatment. SAMs on various substrates offer a flexible method for synthesizing inorganic thin films (such as TiO2,11 ZnO12 and FeOOH13) at low temperature in aqueous solution. Appropriate SAMs can promote the heterogeneous nucleation of solid on the SAMs surface.14 But once the initial few nanometers of films have formed, the effects of SAMs on film growth would not be expected. Rather, solution chemistry and temperature will significantly affect the growing films through controlling the supersaturation degree of solution.15 The differences in pH, * Corresponding author. Tel: +86-21-52413122. Fax: +86-2152412516. E-mail: [email protected].

temperature, and solution composition have large effects on the solution supersaturation. Although there have already been a few studies about the preparation of sulfide crystals/films on SAMs.16 The influence of the above factors on the nucleation and growth of sulfide films were hardly concerned in the reported literature. So it is necessary to discuss the effects of solution conditions, such as solution pH and temperature, on the formation process of Bi2S3 films in this work. Investigation of the mechanisms of nucleation and growth of Bi2S3 thin films may provide valuable information for making desired thin films. Experimental Section SAM Preparation. P-type single-crystal Si (100) wafer (Wafer Works (Shanghai) Corp.; resistivity 20-40 Ωcm) employed as the substrate was cut into 20 × 20 mm2 pieces and cleaned ultrasonically in acetone, ethanol, and deionized water. After drying, the substrates were immersed in piranha solution (a 30:70 mixture of 30% H2O2 and concentrated H2SO4) at 80 °C for 30 min. They were then washed by sonication in deionized water. Through the surface oxidation, the electrical resistivity of silicon substrate is higher than 106 Ω cm. SAMs of (3-mercaptopropyl) trimethoxysilane (MPTMS; Alfa Aesar) grafted onto the silicon oxide surface were prepared by the immersion of clean Si substrate in the absolute ethanol/deionized water mixed solution (9:1 by volume) containing 1 vol % MPTMS. A few drops of glacial acetic acid were added as a catalyst. After immersion the substrates were rinsed with ethanol to remove residual reagents, followed by drying under vacuum. Synthesis of Bi2S3 Thin Films. Bi2S3 thin films were synthesized from an acidic bath solution, which was prepared by mixing 10 mL Bi(NO3)3 solution (0.1 M) and 60 mL thiourea (Tu) solution (0.1 M). Thiourea was used as both sulfide ion source and complexing agent. The solution of 0.1 M bismuth nitrate was prepared by dissolving 9.71 g Bi(NO3)3‚5H2O in 40 mL concentrated nitric acid and then diluting the volume to 200 mL with deionized water. The pH value of bath solution was adjusted by the addition of a few drops of NH3‚H2O (0.1

10.1021/jp065268c CCC: $33.50 © 2006 American Chemical Society Published on Web 11/03/2006

Bismuth Sulfide Thin Films M) or HNO3 (0.1 M). All of the reagents were analytical grade and used without further purification. The as-prepared substrates with MPTMS SAMs were immersed in the middle of the solution, where they were held with the SAM side down to prevent the particles formed in the solution from accumulating on the substrate surface. The solution was kept covered to prevent the evaporation of water. The nucleation and growth of Bi2S3 thin films were allowed to proceed without shaking for different durations (1-72 h) at different temperatures (4070 °C). At the end of the experiments, the films were removed from the bath, rinsed carefully with deionized water, and then dried naturally. Film Characterizations. The elemental compositions of the as-prepared MPTMS SAMs and Bi2S3 thin films were determined by X-ray photoelectron spectroscopy (XPS) performed on a Microlab 310F Scanning Auger Microprobe using nonmonochromatic Mg KR X-rays as the excitation source and C1s as the reference line. The amount of Bi2S3 on the substrate was achieved by dissolving Bi2S3 from known areas in concentrated nitric acid. The concentration of Bi3+ was determined using inductively coupled plasma atomic emission spectrometry (ICPAES; VARIAN) and converted to milligrams of Bi2S3/cm2. The crystalline structures of Bi2S3 thin films were analyzed by an X-ray diffractometer (XRD; Rigaku RINT 2000) operating with Cu KR radiation at 40 kV/40 mA. The morphology of Bi2S3 thin films was characterized by field-emission scanning electron microscopy (FESEM; JSM 6700F, JEOL). The number density of prism-like Bi2S3 crystallites and their edge width of cross section were estimated by averaging the data from SEM photographs. Samples used to examine the micrograph of the contact surface of Bi2S3 films with the substrate were obtained by using conductive adhesives to adhibit the Bi2S3 thin films. A high-resolution transmission electron microscope (HRTEM; JEM-2100F, JEOL) and selected-area electron diffraction (SAED) were used to determine the detailed crystalline structures. TEM samples were obtained by carefully touching the substrate surface with stainless steel stubs. The dark electrical resistivity of Bi2S3 thin films was measured by a four-point probe method in van der Pauw configuration with an Accent HL5500 Hall System at room temperature. Silver paste was applied to provide ohmic contact with the Bi2S3 thin films. Results and Discussion XPS Analysis of SAMs and Films. Chemical attachment of trimethoxysilane SAMs on silicon substrate is based on the reaction of the organosilanes with the oxygen-containing surfaces resulting in the formation of covalent bonds.17 In the presence of water condensation occurred among the MPTMS molecules, leading to cross linkages. Figure 1 shows the immersion time dependence of the thiol coverage on the silicon substrate. The molar ratios of S/Si were obtained from the XPS measurements. The substrates must be handled with great care because any surface contamination may affect the results, so the removal of substrates from vacuum was just prior to the performance of XPS measurements. As shown in Figure 1, there was an initial region of the fast growth of S/Si ratio, after which a slight growth was observed. Near saturation coverage was obtained after 48 h condensation, suggesting the full coverage of MPTMS SAMs on the silicon substrate. Thus, in the following experiments the synthesis of Bi2S3 thin films were performed on the substrates that had been immersed for 48 h in the MPTMS solution. XPS was also employed for the evaluation of the composition and purity of the obtained thin films. The typical wide-scan

J. Phys. Chem. B, Vol. 110, No. 47, 2006 24055

Figure 1. S/Si ratio as a function of the immersion time for the silicon substrate with MPTMS SAMs.

XPS spectrum of as-prepared thin films growing on the MPTMS SAMs is shown in Figure 2a, in which all peaks can be assigned to C, O, Bi, and S elements. The binding energy of the C(1s) transition was used as a reference to standardize the binding energy of other elements. The oxygen peak may be attributed to the O2, CO2, or H2O adsorbed onto the samples from the atmosphere. Two strong peaks at 158.5 and 163.8 eV in the high-resolution XPS spectra (Figure 2b) are assigned to Bi(4f7/2) and Bi(4f5/2), respectively. The peak at 225.8 eV corresponds to the S(2s) transition (Figure 2c). The ratio of peak integrations at the two regions (Bi(4f) and S(2s)) was 0.70, indicating the formation of Bi2S3 films with bismuth excess. The XPS results of the Bi2S3 thin films prepared under different solution conditions demonstrate that the chemical composition did not seem to be relevant to reaction time and solution temperature. Films Formed on MPTMS SAMs. The nucleation and growth of Bi2S3 thin films are affected by the supersaturation degree that can be controlled by varying solution conditions. In this research, the effects of solution temperature and pH on the nucleation and growth of Bi2S3 films were examined in detail. Figure 3 shows the growth rate of Bi2S3 films on the substrate at different temperature and pH. It is shown that after the induction period the film growth was initially linear with time followed by a nonlinear region of much lower growth rate. The increase of solution temperature resulted in higher growth rate and shorter inducing time. In the case of initial pH 1.12 or 0.47, the amount of Bi2S3 on the substrate with MPTMS SAMs nearly reached a maximum after 18 or 72 h in the solution at 70 °C, respectively. The microstructure of Bi2S3 thin films prepared at different pH also exhibited an apparent difference. As shown in Figure 4, the Bi2S3 films consisted of compact tetragonal columnar crystallites at pH 0.47. Comparatively, in the case of pH 1.12 the Bi2S3 particles on the substrate exhibited near-spherical morphologies. The Bi2S3 films formed at pH 1.12 were not dense and continuous as well as the films fabricated at pH 0.47. In the present work, we focus on the preparation of Bi2S3 films with continuous and compact microstructure, so in the next section we pay more attention to the Bi2S3 films fabricated in the solution at pH 0.47. XRD measurements (Figure 5) of the Bi2S3 thin films prepared in the solution of pH 0.47 at 40 °C demonstrate that the Bi2S3 films exhibited good crystallinity despite lowtemperature synthesis conditions. All of the peaks could be indexed as the orthorhombic lattice of Bi2S3 (bismuthinite). The degree of c-axis orientation (F) was evaluated using the

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Figure 2. Typical XPS spectra of the Bi2S3 thin films prepared at pH 0.47, (a) wide-scan spectrum, (b) high-resolution at bismuth region (Bi(4f)), and (c) sulfur region (S(2s)).

Figure 3. Amount of Bi2S3 on the substrate as a function of the reaction duration, (a) at pH 1.12, the open squares, circles, and triangles represent the Bi2S3 films with final thickness of 4.21, 5.57, and 6.82 µm, respectively, (b) at pH 0.47, the closed squares, circles, and triangles represent the Bi2S3 films with final thickness of 2.08, 2.36, and 2.43 µm, respectively.

Figure 4. (a) SEM micrographs of Bi2S3 thin films (a) with thickness 5.57 µm prepared in the solution of pH 1.12 at 55 °C after 20 h reaction and (b) with thickness 2.29 µm prepared in the solution of pH 0.47 at 55 °C after 72 h reaction.

Lotgering method18 taking into account the following diffraction peaks: (220), (130), (211), (221), (240), (002), and (351).

P - P0 1 - P0

(1)

∑ I(001) ∑ I(hkl)

(2)

F)

P)

where P is calculated for the oriented sample and P0 is P for the nonoriented sample (JCPDS Card 17-0320). As shown in Figure 6, the degree of c-axis orientation of Bi2S3 thin films was enhanced with increasing film thickness or reaction time and reached 18.4% after 72 h compared with the randomly oriented specimen. The unit cell constants determined from the

XRD pattern are a ) 11.117 Å, b ) 11.258 Å, and c ) 3.982 Å. Compared with the standard data (a ) 11.149 Å, b ) 11.304 Å, and c ) 3.981 Å, JCPDS Card 17-0320), it was found that the preferential growth along the c axis brings compressive stress along the a and b axes. The detailed crystal structure of the Bi2S3 crystals growing on the MPTMS SAMs was evaluated by TEM and SAED. The inset of Figure 7a is consistent with a crystal elongated in the [001] direction with an electron beam incident along the [11(-)0] zone axis, and as a result the {001} and {110} reflections appeared in the SAED pattern perpendicular and parallel, respectively, to the [001] direction (the long axis of the Bi2S3 crystallite). The observed lattice spacings of 0.40 and 0.79 nm (Figure 7b) correspond to the (001) and (110) planes of orthorhombic Bi2S3, respectively. Both the HRTEM and SAED pattern show that the crystallites grew along the [001] direction. The formation of prism-like morphology is ascribed

Bismuth Sulfide Thin Films

J. Phys. Chem. B, Vol. 110, No. 47, 2006 24057 ability of the aqueous medium to supply the growing Bi2S3 films with materials. The above marked differences in the morphology and growth behavior are related to the solution supersaturation, which is varied by changing the solution pH and temperature. The effect of solution conditions on the supersaturation degree can be understood by considering the reaction process. The nucleation and growth of Bi2S3 thin films are based on the slow release of Bi3+ and S2- ions in the solution. Thiourea is used as both a sulfur source and a ligand, which can form complexes with Bi3+:

Bi3+ + nTu T [Bi(Tu)n]3+

Figure 5. XRD patterns of Bi2S3 thin films prepared at 40 °C for different duration, (a) 24 h, (b) 36 h, (c) 48 h, (d) 60 h, and (e) 72 h. The corresponding thicknesses are 1.15, 1.51, 1.69, 1.83, and 1.97 µm, respectively. Randomly oriented powder pattern of bismuthinite (JCPDS Card No. 17-0320) is shown for comparison.

(3)

Our observation confirms that bismuth nitrate and thiourea can dissolve easily in aqueous solution to form a yellow solution, indicating the formation of Bi-Tu complexes. Upon heating, thiourea is attacked by the strong nucleophilic O atoms of H2O molecules leading to the weakening of the CdS double bonds, which will be broken to produce H2S slowly.20

H2NCSNH2 + 2H2O f 2NH3 + CO2 + H2S

(4)

In an aqueous medium H2S dissociates as21

H2S T HS- + H+ with equilibrium constant K1 ) 10-7 (5) HS- T S2- + H+ with equilibrium constant K2 ) 10-17 (6) It is clear from the above reactions and their equilibrium constants that the predominant species in the solution will be the HS- ions, whreas the S2- ion concentration will be kept low. The increase of pH value could facilitate the forward reaction, inducing the higher S2- ion concentration. The newly formed S2- will react with Bi3+ released from the Bi-Tu complex to produce Bi2S3 nuclei:22 Figure 6. Intensity of (002) peak and c-axial orientation of Bi2S3 thin films obtained from the solution of pH 0.47 at 40 °C as a function of film thickness.

2[Bi(Tu)n]3+ + 3S2- f Bi2S3 + nTu Ksp,Bi2S3 ) 5.01 × 10-62 (7) The overall reaction of Bi2S3 formation can be represented as follows:

3H2NCSNH2 + 2Bi3+ + 6H2O f 6NH3 + 3CO2 + Bi2S3 + 6H+ (8)

Figure 7. Typical (a) TEM, SAED pattern and (b) HRTEM of Bi2S3 crystallites growing on the substrate. Samples were obtained from the Bi2S3 thin films prepared in the solution of pH 0.47 at 40 °C after 72 h reaction.

to the preferential growth of Bi2S3 crystals. The stronger covalent bond between the planes perpendicular to the c axis facilitates a higher growth rate along c axis, whereas the growth in the horizontal direction is much slower due to the weaker van der Waals bonding between the planes perpendicular to the a axis.19 Nucleation and Growth Mechanism of Thin Films. Solution chemistry and temperature have a strong influence on the

Therefore, under heating condition the Bi-Tu complexes underwent a decomposition process for a certain period to produce Bi2S3. The experiments showing species depletion in solution (solution color from yellow to colorless) suggest that the Bi2S3 film growth occurred by consumption of ions and their incorporation into a solid lattice. The release process of sulfur species from thiourea, as well as dissociation of Bi-Tu precursors, could control the solution supersaturation, which further affects the nucleation and growth of Bi2S3 films. According to the usual theory of crystal growth,23 no nucleation occurs in a solution with low supersaturation, homogeneous nucleation occurs at high supersaturation, and films formation from heterogeneous nucleation is possible in the intermediate supersaturation region. In our works, the supersaturation degree is controlled by the solution temperature and pH value. Because eq 4 is an endothermic reaction, when the solution pH value is kept fixed the increase of temperature can promote the formation of H2S, and increase the supersatu-

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Figure 8. Numerical densities of Bi2S3 crystallites on the MPTMS SAMs in the solution of pH 0.47 as a function of the reaction duration.

ration degree, resulting in high nucleation and growth rate (as shown in Figure 3). In contrast, at lower temperatures, the nucleation and growth of Bi2S3 are suppressed, which leads to decreased growth rates of Bi2S3 thin films. The decline of the growth rate with the reaction time was attributed to a reduction in supersaturation degree when the reaction progressed and depleted species from solution. It is also shown in Figure 3 that the induction time, τ, decreased with the increase of solution temperature. The induction period occurs because there is an activation energy required for nucleation, which depends on the interfacial energy for formation of the nuclei and the solution supersaturation, S.13a The activation energy can be decreased by the increase of solution supersaturation. An expression for the induction time as described by classical nucleation is24

τ ) A exp

(

βν2γ3 (kT)3(ln S)2

)

(9)

where β is a shape factor, ν is the molecular volume, γ is the solid/liquid interfacial tension, k is the Bolzmann constant, and T is the absolute temperature. Thus, the increase of solution temperature or supersaturation results in a shorter induction period in the nucleation stage of Bi2S3. As indicated in Figure 4, solution pH has a great effect on supersaturation degree, which can induce an absolutely different microstructure of Bi2S3 thin films. At pH 0.47, the supersaturation degree was low, resulting from the high H+ concentration that suppressed the generation of Bi2S3, so the Bi2S3 thin films were formed mainly by heterogeneous nucleation. However, the degree of supersaturation was high at pH 1.12 because a low H+ concentration promoted Bi2S3 generation as indicated by eq 8. Many homogeneously nucleated Bi2S3 particles could be observed in the solution, and the growth of Bi2S3 thin films was via the attachment of homogeneous nuclei. Therefore, at pH 0.47 dense and continuous Bi2S3 thin films were obtained due to the heterogeneous nucleation process, which produced ion-by-ion growth. This growth pattern is more space-filling than the growth by the attachment of particles. Figure 8 depicts the time dependence of the numerical density and particle size of Bi2S3 crystallites on the MPTMS SAMs at pH 0.47. Here we use the edge width of cross section of a square columnar crystallite to describe the particle size of Bi2S3. When the solution temperature and pH value were kept fixed, the number density increased during a certain period and then decreased, showing a maximum at 24 h. The particle size increased with the reaction proceeding. The variation in the

amount of Bi2S3 (Figure 3b) stemmed mainly from the increase of particle density before 24 h. The decrease of number density after maximum suggests that most of the small crystallites were incorporated into large crystallites. The variation in the amount of Bi2S3 after 24 h resulted mainly from the increase of particle size. SEM micrographs (Figure 9) confirmed the phenomenon of crystallite growth. It is shown that Bi2S3 crystallites became coalesced to form larger crystallites with the increase of reaction time. Thus, at pH 0.47 the prolonged reaction time resulted in more continuous Bi2S3 thin films. The influence of solution temperature on the microstructure of Bi2S3 thin films is shown in Figure 10, where the samples were prepared at pH 0.47 after 24 h reaction. It is obvious that the increase of solution temperature gave rise to a high numerical density of Bi2S3 crystallites. This variation can be explained by taking into account the Bi2S3 nucleation stage. On the assumption that the nucleation phenomenon obeys the Bolzmann distribution law,23a the rate of nucleation, dN/dt, where N is the number density of nuclei, is proportional to the probability to overcome the activation energy by thermal fluctuation and thus obey the relation

dN/dt ∝ exp(-∆Gs/kT)

(10)

where ∆Gs is the free energy for heterogeneous nucleation on the substrate, which decreases with the increase of solution temperature. When the solution temperature, T, was high, a large density of nuclei on the MPTMS SAMs was obtained after the same duration and then provided more nucleation sites for Bi2S3 crystallites. Thus, Bi2S3 films displayed a more compact microstructure with increasing solution temperature after the same reaction time. Electrical Property. Previous studies have shown that Bi2S3 thin films synthesized by chemical solution methods exhibited high electrical resistivity mainly due to poor crystallinity, high grain boundary density, and discontinuities of the films. In some research, annealing of Bi2S3 films was carried out to decrease electrical resistivity.10 In the present work annealing was not performed, but it is reasonable to expect low resistivity in the Bi2S3 thin films based on the SAMs. The effect of synthesis conditions on the dark resistivity at room temperature, as well as the carrier concentration and mobility, is given in Table 1. The carrier concentrations (1015-1017 cm-3) are higher than the intrinsic carrier concentration (∼107 cm-3), which is attributed to the bismuth excess in the Bi2S3 films prepared at pH 0.47. The bismuth excess is related to the formation process of Bi2S3 phases. To be specific, Bi2S3 thin films are formed by the heterogeneous nucleation mechanism, which is based on the slow release of S2- ions from Tu. The low pH of the solution would suppress the generation of S2- according to eqs 4-6. The heterogeneous nucleation occurs only on the SAM surface and depletes the near species. There is thus a difference in sulfur species concentrations between the vicinity of the MPTMS SAMs and the inner solution. This difference results in the films growing on the SAMs with sulfur deficiency or bismuth excess. Besides, the negative Hall coefficient from Hall measurement indicates that the conductivity type is n-type, which is consistent with the bismuth excess. In Table 1, for the Bi2S3 thin films fabricated at pH 1.12 the accurate resistivity was too large to be determined, whereas, for the Bi2S3 films prepared at pH 0.47, lower electrical resistivity was observed with increasing solution temperature or reaction time. The lowest resistivity of the prepared Bi2S3 films is on the order of 10-2 Ω cm, which is much lower than the reported resistivity of Bi2S3 thin films prepared by traditional

Bismuth Sulfide Thin Films

J. Phys. Chem. B, Vol. 110, No. 47, 2006 24059

Figure 9. SEM micrographs of Bi2S3 thin films fabricated in the solution of pH 0.47 at 70 °C after (a) 48 h, (b) 60 h, and (c) 72 h reaction.

Figure 10. SEM micrographs of Bi2S3 thin films prepared in the solution of pH 0.47 at (a) 40 °C, (b) 55 °C, and (c) 70 °C after 24 h reaction.

TABLE 1: Effects of Solution pH, Temperature, and Reaction Time on the Electrical Properties of Bi2S3 Thin Films at Room Temperature solution condition sample number

pH

t (h)

T (°C)

thickness (µm)

1 2 3 4 5 6 7

0.47 0.47 0.47 0.47 0.47 0.47 1.12

24 24 24 48 60 72 20

40 55 70 70 70 70 70

1.15 1.56 1.87 2.32 2.40 2.43 6.82

chemical solution methods.6b,8,9 Because the elemental compositions of the obtained Bi2S3 thin films are exactly the same from the XPS results, the difference in the electrical properties of Bi2S3 thin films corresponds mainly to the changes in their crystallinity and microstructure. As we have discussed in the previous section, at pH 0.47 the increase of solution temperature or reaction time resulted in higher crystallinity, more continuous and compact microstructure, and consequently lower resistivity. To clearly illustrate the microstructure of Bi2S3 thin films prepared at pH 0.47, SEM was employed to investigate the cross section of Bi2S3 films. Figure 11a shows the typical SEM micrograph for the cross-sectional profile of the Bi2S3 thin films obtained from the solution of pH 0.47 at 70 °C after 60 h reaction. The Bi2S3 films exhibited uniform and continuous morphology with the thickness of 2.40 µm. The enlarged SEM micrograph of the cross section (Figure 11b) demonstrates that the films consisted of closely packed crystallites, which grew continuously from the bottom to the surface of Bi2S3 thin films. Figure 11c shows the SEM morphology of the contact surface of Bi2S3 films with the substrate. This extremely smooth surface had a good varnish, indicating that the Bi2S3 films were greatly coalescent without evident gaps. The crystallites with anisotropic columnar structures grew coadjacently. Apart from good crystallinity, continuous microstructure and bismuth excess, the low

carrier concentration (cm-3)

mobility (cm2 V-1 s-1)

resistivity (Ωcm)

5.63 × 1015 1.11 × 1016 4.88 × 1016 2.23 × 1017 2.56 × 1017 2.86 × 1017

54 107 480 1328 1489 1528

20.56 5.256 0.267 0.0211 0.0164 0.0143 >106

resistivity of the Bi2S3 thin films is also related to the anisotropic Bi2S3 crystals. Cantarero et al. 2b have proved that the electron Hall mobility in the c-axis direction is higher than that in the a-axis direction due to electron effective-mass anisotropy. In this work, the prepared Bi2S3 films are composed of crosslinkage c-axis-oriented crystallites; thus, the conductive property is enhanced. Conclusions Highly crystalline Bi2S3 thin films with low electrical resitivity were prepared successfully on the MPTMS SAMs in an aqueous solution of bismuth nitrate and thiourea at low temperatures (40-70 °C). The growth behavior and microstructure of Bi2S3 films could be controlled by careful manipulation of solution parameters and reaction time. Higher solution temperature increased the supersaturation, resulting in a higher growth rate and a shorter induction time. The growth of discontinuous Bi2S3 films via attachment of nuclei occurred in the solution of pH 1.12 whose supersaturation degree was high as a result of the low concentration of H+. In contrast, at pH 0.47 the Bi2S3 films grew via ion-by-ion incorporation and exhibited continuous and compact microstructure, resulting from the heterogeneous nucleation. The c-axis orientation of Bi2S3

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Figure 11. Typical SEM micrographs for (a) the cross-sectional profile of the Bi2S3 thin films, (b) the enlarged micrograph of parts a and c the contact surface, samples obtained from the solution of pH 0.47 at 70 °C after 60 h reaction.

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