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Direct Precursor Conversion Reaction for Densely Packed Ag2S Nanocrystal Thin Films Qun Tang, Hyun Jae Song, Hye Ryung Byon, Hyun Jin Yang, and Hee Cheul Choi* Department of Chemistry, Pohang UniVersity of Science and Technology, San 31, Hyoja-Dong, Nam-Gu, Pohang, South Korea, 790-784 ReceiVed August 24, 2006. In Final Form: December 12, 2006 Successful realization of highly crystalline and densely packed Ag2S nanocrystal (NC) films has been achieved by directly converting precursor molecules, Ag(SCOPh), on preheated substrates. When an aliquot of Ag(SCOPh) solution dissolved in trioctylphosphine (TOP) is applied on preheated solid substrates at 160 °C, such as SiO2/Si, H-terminated Si, and quartz. Ag2S NC thin films have been formed with instant phase and color changes of the precursor solutions from pale yellow homogeneous solution to black solid films. The average diameter of individual Ag2S NCs forming thin films is ca. 25 nm, as confirmed by examining both isolated Ag2S NCs from thin films and as-made thin film samples by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Powder X-ray diffraction (XRD) pattern shows that the synthesized Ag2S NCs have well-defined monoclinic acanthite phase. Direct precursor conversion process has resulted in densely packed Ag2S NCs with reduced interparticle distances owing to efficient removal of TOP during the reaction. Compared to the devices fabricated by the drop-coating process, Ag2S thin film devices fabricated by direct precursor conversion process have shown a ca. 300-fold increased conductance. Such Ag2S NC devices have also displayed reliable photoresponses upon white light illumination with high photosensitivity (S ≈ 1).
Introduction Among many prospective applications that semiconductor nanocrystals (NCs) can be devoted to, NC-based electronic thin film devices (TFDs) have been widely studied for LED, solar cells, electro-optic modulators, and field effect transistor (FET) applications.1 The conventional structure of a NC-TFD is composed of an array of NC thin film squeezed between lithographically defined two electrodes.2 It is widely accepted that simple transfer of NCs synthesized in the solution phase using spin- or drop-coating onto solid substrates is the ideal method to fabricate TFDs because such NCs show high crystallinity and narrow size distribution. Despite technical simplicity in the fabrication of NC-TFDs over other types of nanomaterial-based electronic devices, for example onedimensional carbon nanotube or Si and Ge nanowire based field effect transistors that require precisely controlled positioning and patterning, most of the prototypical NC-TFDs have been struggling from relatively low conductance. The central reason for the low conductance is the presence of numerous junctions of NCs connected through molecular capping agents which determine interparticle distances among NCs. Moreover, high packing density of NCs is required because cracks and defects that might be introduced during the coating or drying processes will definitely decrease the transport efficiency.3 Other than the crystallinity of NCs, numerous critical factors still should be carefully controlled to acquire the optimal conditions to obtain * To whom correspondence should be addressed. E-mail: choihc@ postech.edu. (1) (a) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (b) Yoffe, A. D. AdV. Phys. 2001, 50, 1. (2) (a) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506. (b) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86. (c) Konstantos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Nature 2006, 442, 180. (d) McDonald, S. A.; Konstantaos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (3) (a) Banerjee, I. A.; Yu, L. Matsui, H. Proc. Nat. Acad. Sci. 2003, 100, 14678. (b) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55.
high-quality NC films. For example, kinds of solvents, surface functional groups, and annealing temperature should be collectively decided since they affect evaporation rate, nucleation patterns, and interparticle distances among NCs, respectively, which eventually determine the quality of NC films.4-7 Recently, utilization of single precursor molecules has drawn attentions since they provide a simpler pathway to the synthesis of hetero-component semiconductor NCs than the conventional solution-phase synthetic methods that use two individual precursor molecules, for example dimethyl cadmium or cadmium oxide and selenium metal for the synthesis of CdSe NCs. Other groups and we have recently reported successful preparation of CdSe, ZnSe NCs,8 and Ag2S semiconductor NCs9 using single precursor molecules. The key strategy of Ag2S NC formation9a is to instantly supply enough thermal energy to single precursor molecules, Ag(SCOPh) to be fragmentized into [Ag] and [AgS] simultaneously by injecting Ag(SCOPh) in a preheated solvent. To the best of our knowledge, there is no report about direct thin film formation of NCs using single precursor molecules. Here, we show that highly crystalline and fairly homogeneous Ag2S NC thin films are straightforwardly created on various preheated solid substrates via direct precursor conversion process performed without serious consideration of the factors that (4) (a) Murray, C. B.; Kagan, C. R. Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (b) Mitzi, D. B.; Kosbar, L. L;. Murray, C. E.; Copel, M.; Afzali, A. Nature 2004, 428, 299. (c) Wehrenberg, B. L.; Yu, D.; Ma, J.; Sionnest, P. G. J. Phys. Chem. B 2005, 109, 20192. (d) Sullivan, S. C.; Steckel, J. S.; Woo, W. K;. Bawendi, M. G.; Bulovic. V. AdV. Funct. Mater. 2005, 15, 1117. (5) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871. (6) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12, 1526. (7) (a) Yu, D.; Wang, C.; Guyot-Sionnest, P. Science 2003, 300, 1277. (b) Jarosz, M. B.; Poter, V. J.; Fisher, B. R.; Kastner, M. A.; Bawendi, M. G. Phys. ReV. B 2004, 70, 195327. (8) (a) Ludolph, B.; Malik, M. A.; O’Brien, P.; Revaprasadu, N. Chem. Commun. 1998, 1849. (b) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, S. Chem. Mater. 2002, 14, 1576. (9) (a) Tang, Q.; Yoon, S. M.; Yang, H. J.; Lee, Y.; Song, H. J.; Byon, H. R.; Choi, H. C. Langmuir 2006, 22, 2802. (b) Lim, W.; Zhang, Z.; Low, H.; Chin, W. Angew. Chem., Int. Ed. 2004, 43, 5685.
10.1021/la062497h CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
Direct Precursor ConVersion Reaction for Thin Films Scheme 1. Schematic Diagrams of Direct Precursor Conversion Process by Which Densely Packed Ag2S Nanocrystal (NC) Films Are Formed (top) and Drop-Coating Process Depositing Presynthesized Ag2S NCs Followed by Solvent Drying (bottom)
Langmuir, Vol. 23, No. 5, 2007 2801 mask. The electrode gap of both samples was 100 µm. Currentvoltage (I-V) characteristic curves and photocurrent responses were measured using a semiconductor analyzer (Keithley SCS4200). For photocurrent response measurements, white light (tungsten-halogen, 30 W) was illuminated while a constant bias was applied on the devices. The photocurrent was recorded by turning on and off the light sources at every 50 s.
Results and Discussion
should be contemplated in drop- or spin-coating methods. A similar approach, called ‘solution growth technique’, has been previously suggested by which films of ZnS and CdS NCs are directly formed on solid substrates when individual molecular precursors are sequentially introduced into a reaction vessel in the presence of target solid substrates and are heated up to the designated reaction temperature.10 However, it has still been challenging to achieve highly crystalline NCs with homogeneous particle sizes. Experimental Section Synthesis of Ag2S NCs on Solid Substrates. 1. Direct ConVersion of Precursor Molecules. The single precursor molecule, silver monothiobenzoate (Ag(SCOPh)), was synthesized as described in reference 11. A degassed solution of 10 mM Ag(SCOPh) was prepared by dissolving it in 0.2 mL of trioctylphosphine (TOP). Si wafers (1 cm × 1 cm) on which 500 nm thick SiO2 layers were deposited (denoted as SiO2/Si) and quartz wafers were cleaned by thorough rinsing with acetone, isopropanol, and deionized water. Hydride-terminated Si wafers were prepared by immersing Si wafers containing native oxide layers in 2.5% HF solution for 2 min and dried using N2 flow.12 The substrate was placed in a round-bottom flask and then heated up to 160 °C under a N2 atmosphere. As soon as the temperature reached 160 °C, 10 µL of the prepared Ag(SCOPh) solution was injected using a microsyringe to be spread on the substrate surface and kept for 10 min (top of Scheme 1). 2. Drop-Coating Process. A Ag2S NC suspension was synthesized from Ag(SCOPh) precursor molecules by hot-injection method as described earlier.9 Several droplets of the suspension containing Ag2S NCs in methanol or TOP were applied onto the surface of SiO2/Si substrates at room temperature. The samples were first dried in vacuum and then heated on a hot plate at 160 °C for 10 min under the protection of nitrogen in order to remove TOP molecules13 (bottom of Scheme 1). The morphology of as-deposited and annealed NC films was studied using SEM. The XRD pattern was recorded using a Rigaku D-Max 2000 diffractometer (Cu KR radiation). Fabrication of a Ag2S NC Thin Film Device. In order to investigate electrical conductivity of the prepared NC-TFDs, electrode metal pads were addressed on the preformed Ag2S NC thin films by evaporating Cr (15 nm) followed by Au (25 nm) through a shadow (10) (a) Lee, J.; Tsakalakos, T. Nanostruct. Mater. 1997, 8, 381. (b) Lee, J.; Lee, S.; Cho, S.; Kim, S.; Park, I. Y.; Choi, Y. D. Mater. Chem. Phys. 2002, 77, 254. (11) Savant, V. V.; Gopalakrishnan, J.; Patel, C. C. Inorg. Chem. 1969, 9, 748. (12) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2000, 16, 10359. (13) Annealing nanocrystal solids at low temperature has been known to efficiently reduce the distance between adjacent NCs and enhance the electron transport properties. See also: Drndic, M.; Jarosz M. V.; Morgan, N. Y.; Kastner, M. A.; Bawendi, M. G. J. Appl. Phys. 2002, 92, 7498.
As described in our previous report, Ag(SCOPh) molecules are dissociated into [Ag] and [AgS] fragments when the precursor molecules are injected into a preheated solvent system at 160 °C. The simultaneously generated [Ag] and [AgS] fragments are then combined to form Ag2S NCs. Basically the same idea has been applied for the formation of Ag2S NC thin film on solid substrates. Upon the deposition of an aliquot of precursor solution on a SiO2/Si substrate preheated at 160 °C in N2 atmosphere, Ag2S NC thin film has been formed with abrupt color and phase changes of the precursors. The originally pale yellow and homogeneous precursor solution turns into a black solid film. Figure 1a shows a representative SEM image (top-view) of Ag2S NC films formed by direct conversion of Ag(SCOPh) precursor molecules on a preheated SiO2/Si substrate. Under the detailed SEM investigations, it turns out that the size of Ag2S NCs forming the thin film is quite homogeneous and the density is also high. The cross-section view of the same sample shows that the thickness of the film is ca. 3 µm and confirms that the density and homogeneity of the thin film is reasonably high (Figure 1b). It has been also verified that the quality of the film is almost uniform over all of the area which is ca. 0.2 cm2. There are several small holes which might be originated from the rough cleavage of the sample when they are cut using a diamond saw or from the abrupt release of TOP gas during the reaction. The release of TOP during the reaction has been confirmed by X-ray photoelectron spectroscopy (XPS) studies. While the Ag- and S-related peaks from Ag2S NCs are clearly observed from the survey spectrum (Figure 1c), the relative intensity of the peak from P 2p (130 eV) is very small (inset of Figure 1c). It should be noted that most of NCs decorated with TOP molecules on their surfaces show highly resolved P 2s and P 2p peaks with strong intensities.14 The crystallinity and phase identification of the Ag2S NC film were studied using a powder XRD. As shown in Figure 1d, the film shows highly crystalline structures with well-resolved diffraction peaks which are indexed as a monoclinic acanthite phase Ag2S (JCPDS 14-0072). The low background intensity indicates that the portion of amorphous species is fairly low. This points out that direct conversion process of Ag(SCOPh) on a preheated substrate is efficient to synthesize highly crystalline Ag2S NCs. Due to the initial high temperature of substrates, Ag2S NCs could be severely agglomerated (or sintered) into lager size grains as soon as they are individually formed. Since the formation of grains and their boundary sizes is one of the critical factors that may affect on their electrical transport properties (vide infra), we have further investigated about the homogeneity and the size distribution of individual Ag2S NCs formed on a SiO2/Si substrate. As shown in Figure 2a, Ag2S NCs desorbed from a substrate by using ultrasonication in acetone show well-defined spherical shapes and no evidence of the larger agglomerated grain formation. Moreover, the size distribution of such Ag2S NCs has (14) Yang, H. T.; Shen, C. M. Wang, Y. G.;. Su, Y. K; Yang, T. Z.; Gao, H. J. Nanotechnology 2004, 15, 70.
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Figure 1. Representative SEM images of (a) top and (b) cross-section views of densely packed Ag2S NCs prepared by direct precursor conversion process. Ag2S NC films show almost trioctylphosphine (TOP)-free and highly crystalline structures as confirmed by (c) X-ray photoelectron spectroscopy (XPS) spectra and (d) powder X-ray diffraction (XRD) patterns, respectively. The inset in (c) shows a magnified view of the region where phosphorus peaks appear.
Figure 2. (a) Typical TEM image of Ag2S NCs isolated by ultrasonication of Ag2S NC thin films prepared by the direct precursor conversion process. (b) Size distribution of Ag2S NCs obtained from 304 individual NCs in (a). The average diameter is ca. 25 nm.
been statistically obtained from 304 individual NCs, and it spans from 20 to 30 nm with average diameter of ca. 25 nm (Figure 2b). This value agrees well with the average diameter data of Ag2S NCs obtained from SEM studies. The resulting high-quality and densely packed Ag2S NC thin film could be obtainable only when (1) the nucleation of nanocrystals takes place homogeneously over the wide range of solid surface and (2) interparticular agglomeration is inhibited. Our system seems to satisfy both requirements pretty well. First, the single source precursor molecules, Ag(SCOPh), are homogeneously dissolved in TOP (solvent) which are also well spread on solid substrates under nitrogen atmosphere. This primarily allows homogeneous nucleation of Ag(SCOPh) molecules on the wide range of the substrate. Second, interparticular agglomeration is inhibited mostly at the stage of nucleation by TOP. Although TOP is removed during the reaction by thermal energy supplied from the substrate, the formation of Ag2S thin film occurs (observed by surface color changes) faster than the evaporation of TOP (observed by released white gases). As described earlier, one of the remarkable advantages of Ag2S NC thin film formation via direct precursor conversion process is that this method works adequately without special care of external conditions including specific surface functional groups on substrates which play critical roles in determining
quality of thin films if they are formed by spin- or drop-coating processes. In the case when Ag2S NC thin films are prepared via drop-coating using separately synthesized Ag2S NC suspension on the same SiO2/Si substrates, the film quality turns out to be significantly low (bottom of Scheme 1 and Supporting Information Figure S1). Although there might be room to increase the quality of the film by applying special chemical treatments to the surfaces or by selecting more controlled reaction environments, the direct precursor conversion method can be generally depicted, as it provides much facile and straightforward way for the high-quality Ag2S NC thin film formation. We have additionally confirmed that the direct precursor conversion process results in similar qualities of Ag2S NC films from various substrates such as hydride-terminated Si and quartz (Supporting Information Figure S2). The reason for the similar quality of Ag2S NC films on different substrates seems due to the wetting ability of TOP that is coated well regardless of the types of surfaces,15 i.e., hydrophilic or hydrophobic surfaces, which make those substrates play an equal role during the formation of NC thin films. Another interesting characteristic of Ag2S NC thin films is their chemical and mechanical stability. The films show high chemical stabilities as they still keep their original shapes intact upon soaking them into acetone overnight. As for the mechanical stability, the film is stable enough not to be damaged upon harsh rinsing with various organic solvents but not highly resistant against mechanical scratches such as a finger tip scratch, which is also the same for the all NC thin films fabricated by spin- or dip-coating processes. The reasonably high mechanical stability is attributed to the strong dipole interactions between NCs capped with TOP containing unpaired electrons and substrates, as well as strong noncovalent interactions existing among NCs. In general, the electrical conductivity of NC-TFD is quite lower compared to other one-dimensional semiconductor na(15) When droplets of TOP containing Ag(SCOPh) precursors were dropped on SiO2/Si (hydrophilic) and hydride-terminated Si (hydrophobic) substrate surfaces at 160 °C in N2 atmosphere, the sizes of droplets covering the surfaces were almost identical.
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Figure 3. (a) Schematic view of a two-electrode-type Ag2S NC thin film device. (b) SEM image of the fabricated device. The black and gray regions represent Ag2S NC thin film and Cr/Au metal electrodes, respectively. Representative I-V curves of Ag2S NC thin film prepared by direct precursor conversion process (c) and drop-coating process (d) with a channel gap of 100 µm.
nomaterial-based devices. One of the main reasons for the relatively lower conductivity is the high potential energy barriers originated from the existence of organic capping molecules among NCs and correspondingly defined long interparticle distances. The Ag2S NC film formed via direct precursor conversion process, however, is believed to overcome the aforementioned obstacles because of the instant removal of TOP during the reaction, as described earlier. For the electrical transport property studies, we have fabricated two-terminal electronic devices using Ag2S NC films (Figure 3a). Using a shadow mask, multiple Au electrode pads were defined on Ag2S NC thin films as the channel length between the two electrodes to be ca. 100 µm (Figure 3b). Asdeposited Ag2S NC thin film devices reproducibly show conductance of 1.5-2.0 × 10-4 S‚cm-1 (Figure 3c) which are ca. 300-fold higher than the conductance of Ag2S NC device prepared by drop-coating process (5-6 × 10-7 S‚cm-1, Figure 3d). Note that these average values are obtained from five individually fabricated samples for each measurement. Such increased conductance primarily owes to the increased electronic communication among the NCs. The charge transport in NC solids separated by an insulating capping agent is determined by the exchange coupling energy which can be approximately expressed as
β ≈ exp[-κ(d + δ)] where d is the diameter of NC, δ is the interparticle spacing, and κ is the length scale of the wave function leakage outside the volume of NC.16 According to the equation, the reduction of interparticle distance would greatly enhance the efficiency of electron transport, and it has happened in our direct precursor conversion process as most of TOP molecules are evacuated as soon as NCs are formed. Attempts to reduce the interparticle distance further down via surface-capping molecule exchange reaction with diamine or bidendate hydrazine have failed to (16) (a) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (b) Remacle, F.; Beverly, K. C.; Heath, J. R.; R. D. Levine, J. Phys. Chem. B 2003, 107, 13892.
Figure 4. Photocurrent responses recorded from a Ag2S NC thin film device at constant bias voltage of 5 mV.
increase conductivity.2a,7 This result explains that there is no significant amount of exchangeable TOP molecules on Ag2S NC surface, and consequently, the interparticle distance has already been reached at the minimum level. More importantly, conduction hysteresis from our devices is significantly diminished, which denotes that Ag2S NCs are not only densely packed but also contain less surface dangling bond and defects. The hysteresis is generally originated from numerous electron or hole trap sites in NCs and NC films, and it becomes more significant when NC films have low degree of packing density with inefficient electronic communication. For example, Ag2S NC films prepared by drop-coating show low packing density and low conductivity with high degree of relative hysteresis (Figure 3d). Similar behavior also has been observed when the capping ligands are intentionally removed in order to decrease interparticle distance from drop-coated PbSe NC devices.2a,7 Last, we have investigated photoconductivity of Ag2S NC film devices. As shown in Figure 4, prompt current increases up to ∼50% have been observed when the device is exposed to white light under a constant bias voltage of 5 mV. The device shows similar photoresponse ability after multiple cycles without
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any loss in stability, even after keeping them in ambient air for a month. The relative photosensitivity (S), defined as S ) (Iph - Id)/Id, where Iph and Id are the photocurrent and the dark current, respectively, is close to 1. This value is comparable to the ones generally observed from bulk scale Ag2S film devices, where a secondary annealing process is required to obtain photoresponse properties.17 Considering the fundamental advantages of NCbased thin film which bring unique optical and electrical properties, the substantial enhancement of the photosensitivity of our Ag2S NC thin film device holds promising for the continuous improvements in NC-based thin film devices. In summary, Ag2S NC thin films have been fabricated via a direct precursor conversion reaction. Instant conversion of Ag(SCOPh) molecules on preheated SiO2/Si, H-Si, and quartz substrates allows thin film formation of highly crystalline and densely packed Ag2S NCs with remarkably reduced interparticle distances among NCs which eventually results in improved electrical conductivity, as well as photocurrent responses. With appropriate selections of precursor molecules concerning thermal energy correlation for the selective degradation of precursors, (17) Rodriguez, A. N.; Nair, M. T. S.; Nair, P. K. Semicond. Sci. Tech. 2005, 20, 576.
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the NC thin film formation method that we have developed may permit an enormous chance for the fabrication of various types of high-performance NC-based electronic and optoelectronic devices. Despite our method demonstrating an obvious advantage over spin- or dip-coating process in terms of ease and reproducibility achieving shortened interparticle distances among NCs with reasonably high packing density, the controllability of particle sizes and film thickness should be guaranteed in order for this method to be applied for the practical applications, which may be achieved by systematically investigating the effects of precursor concentration and types of solvent. Acknowledgment. This work was supported by the Basic Research Program of the KOSEF (R01-2004-000-10210-0), Nano/Bio Science & Technology Program of MOST (M10536090000-05N3609-00000), SRC/ERC Program (R11-2000-070070020), Korean Research Foundation (MOEHRD, KRF-2005005-J13103). Supporting Information Available: Images of Ag2S NC films. This material is available free of charge via the Internet at http:// pubs.acs.org. LA062497H
