Near-Infrared Absorption of Monodisperse Silver Telluride (Ag2Te

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Near-Infrared Absorption of Monodisperse Silver Telluride (Ag2Te) Nanocrystals and Photoconductive Response of Their Self-Assembled Superlattices Yu-Wen Liu,† Dong-Kyun Ko,† Soong Ju Oh,† Thomas R. Gordon,‡ Vicky Doan-Nguyen,† Taejong Paik,‡ Yijin Kang,‡ Xingchen Ye,‡ Linghua Jin,† Cherie R. Kagan,†,‡,§ and Christopher B. Murray*,†,‡ †

Department of Materials Science and Engineering, ‡Department of Chemistry and §Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States

bS Supporting Information KEYWORDS: silver telluride, Ag2Te, near-infrared, photoconductivity, semiconductor, nanocrystal, superlattice

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ilver telluride nanocrystals (Ag2Te NCs) have been an enigma showing an anomalous growth process, yielding unusual structure and optical response.1 3 Ag2Te is a little explored member in the silver chalcogenide family1,2,4 that possesses interesting physical properties including a transition from semiconductor to ionic conductor5,6 and large magnetroresistance in nonstoichiometric samples.7 Ag2Te exhibits high electron mobility and low thermal conductivit,y8 which are desirable for thermoelectric applications.9 This physics becomes richer when studied at the nanoscale dimensions comparable to the natural delocalization lengths of the electronic and thermal excitations. Syntheses of Ag2Te nanowires,10 nanotubes,11 nanorods,12 and nanoparticles1 3 have been reported. Although we have synthesized a range of Ag2Te NC sizes, we focused here on the optical and optoelectronic properties of monodisperse 3.1 nm Ag2Te NCs which uniquely exhibit a sharp optical absorption feature at 1154 nm. This wavelength is of particular interest as it falls in a window of transparency for biological tissue.13,14 The energy of the optical resonance shows a strong negative linear temperature dependence but unlike other semiconductor NCs the energy is not tunable with size.15 These NCs also differ from noble metal NCs where the plasmonic peak shifts as a function of the dielectric constant of the surrounding medium.16 These robust spectral responses could enable the use of Ag2Te NCs as a temperature reporter15,17 or a transducer18,19 of infrared energy in biological tissues and chemical environments free of complications stemming from spectral shifts due to the surrounding dielectric constant. The Ag2Te NCs were synthesized via rapid injection of trioctylphosphine-telluride (TOP-Te) into a mixture of silverdodecanethiol and 4-tert-butyltoluene (see the Supporting Information).3 Figure 1a f and g show TEM images and the absorption spectra, respectively, of Ag2Te NCs as a function of growth time. Aliquots retrieved at 10 min after TOP-Te injection show a wide distribution in NC size and shape (Figure 1a) and a broad absorption feature (A in Figure 1g). After 1 h the growth solution shows a bimodal distribution of large (6 10 nm) and small 3.1 nm NCs (Figure 1b) and a distinct absorption peak arising from the small NCs starts to emerge (B in Figure 1g). Large NCs transform to a barrel shaped NCs (Figure 1c) with no distinct absorption peaks (C in Figure 1g), whereas a population of small NCs does not change size further with time. After 12 h r 2011 American Chemical Society

large NCs grow and precipitate leaving behind small monodisperse NCs in solution (Figure 1d and D in g). After 24 h the majority of the large NCs have precipitated and the sharp absorption features arising from the small NCs (E in Figure 1g) predominate in solution. Figure 1f shows that the precipitates are mixtures of tabular and prismatic shapes common in monoclinic minerals.20 The nucleation and growth of Ag2Te NCs differ from the well-established hot-injection mehtods for II VI or IV VI semiconductor NC where NCs continuously grow from the nuclei through focusing and defocusing regime.16,21 This growth also differs from previously reported growths for magic size clusters22 or digestive ripening processes in noble metals.23 For further chemical, structural, and optical characterization, three different sizes of Ag2Te were extracted from the reaction (see the Supporting Information). Figure 2a c shows TEM images of the small (3.1 nm), large NCs (6 10 nm), and the precipitates (prismatic shapes larger than 15 nm in width and tabular shape larger than 35 nm in width), respectively. The chemical analysis using atomic emission spectroscopy indicates the small monodisperse NCs have a large stoichiometric excess of silver (Ag:Te = 2.39:1) with the Ag to Te ratio decreasing with increasing size but remains Ag-rich. This is consistent with NC surface that is terminated with Ag+ ions sites that are coordinated with surfactant molecules where the stoichiometric excess of Ag24,25 varies with the surface-to-volume ratio. Bulk Ag2Te with the stoichiometric excess of Ag shows three polymorphs. The low temperature monoclinic β-phase (optical energy gap ∼0.67 eV) transforms to the face-centered cubic α-phase at 145 °C and to the body-centered cubic γ-phase at 802 °C.26,27 Structural characterization of Ag2Te NCs using powder X-ray diffraction (XRD) is shown in Figure 2d. The diffraction pattern of the precipitates is identical to the low temperature β-Ag2Te (monoclinic, JCPDS: 081 1820), which is consistent with our simulations (see the Supporting Information). Interestingly, for small NCs the XRD pattern deviates from the β-phase better matching the simulated mixtures of 67% γ (BCC, JCPDS 081 1824) and 33% α (FCC, JCPDS 081 1821) phases Received: July 11, 2011 Revised: September 15, 2011 Published: October 10, 2011 4657

dx.doi.org/10.1021/cm2019795 | Chem. Mater. 2011, 23, 4657–4659

Chemistry of Materials

Figure 1. TEM images of Ag2Te NCs obtained at different growth times. (a) 10 min, (b) 1 h, (c) 4 h, (d) 12 h, (e) 24 h, and (f) the precipitates obtained from 24 h of reaction. All scale bars are 50 nm. (g) Absorption spectra of Ag2Te NCs at different growth times.

Figure 2. TEM images of (a) 3.1 nm NCs, (b) 6 10 nm NCs, and (c) precipitates of Ag2Te. The stoichiometry is denoted in the inset. All scale bars are 50 nm. (d) Powder X-ray diffraction (XRD) patterns and the corresponding simulation curves. (e) Absorption spectra obtained from the 3.1 nm and 6 10 nm Ag2Te NCs. (f) Transmission small-angle X-ray scattering (SAXS) data obtained from Ag2Te NCs shown in a and the simulation curve indicates the average size of 3.4 nm with the distribution of 7%.

as shown in Figure 2(d). In the bulk both are high temperature phases, unstable at room temperature.27 However, theory28 and experiment29 have shown phase equilibria in NCs can differ significantly from bulk analogs. This suggests that the γ and α nanocrystalline phases may be captured at the reaction temperature and are at least metastable down to room temperature. The XRD pattern of large NCs, was better fit by a mixture of 67% γ and 33% β phase. Bulk α and γ Ag2Te are ionic conductors with high silver ion mobility at high temperature.30 The optical absorption spectra obtained from small and large Ag2Te NCs are shown in Figure 2e. Large NCs show featureless absorption similar to that seen in monodisperse Ag2S and Ag2Se.1,2 On the other hand, small 3.1 nm Ag2Te NCs with 7% size dispersion (Figure 2f) show multiple distinct absorption peaks reminiscent of discrete energy levels in quantum confined semiconductor NCs. However, no emission was observed possibly due to the quenching from surface traps. To help determine if the optical absorption in Ag2Te NCs is an interband transition (with an energy gap of 1.07 eV) we acquired temperaturedependent optical absorption measurements. A dramatic blue shift is seen as the temperature is lowered (Figure 3a,b),

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Figure 3. (a) Temperature-dependent spectral shift observed in monodisperse 3.1 nm Ag2Te NCs (dEg/dT = 0.25 meV/K) plotted for two separate measurements. (b) Optical absorption as a function of temperature. (c) Absorption spectra of Ag NCs no discernible shift in their plasmonic resonance with temperature. (d) Absorption spectra obtained from 3.1 nm Ag2Te NCs dissolved in solvents with different dielectric constants, shows the absorption is insensitive to the surrounding dielectric medium.

Figure 4. (a) Time-resolved photocurrent switching between on and off states. (b) Photocurrent decay and the fitting curve using the biexponential decay function as described in the text. The inset shows the self-assembled superlattice of monodisperse 3.1 nm Ag2Te NCs used for the photoconductivity measurement.

suggesting transition is rather than a plasmonic resonance.15,31 For a comparison, silver NCs show negligible shift as a function of temperature (Figure 3c). Ag2Te NCs also do not show spectral shift when the dielectric constants of the solvent are changed. No shift is seen in carbon tetrachloride (dielectric constant ε = 2.2), tetrachloroethylene (dielectric constant ε = 2.5) and trichloroethylene (dielectric constant ε = 3.3) (Figure 3d), excluding the possibility of the absorption arising from plasmonic resonance.32 In addition, assuming bulk density of 8.5 g/cm3, the extinction coefficient for the first resonance at 1154 nm was determined to be 0.73  105 cm 1 M 1 (see Supporting Information, Figure S1), comparable to previously reported values for CdSe33 and PbSe24 NCs with similar size. To probe the semiconducting nature of Ag2Te NCs, we performed photoconductivity measurements on Ag2Te NC films treated with 1,2-ethanedithiol. The superlattice film prepared from monodisperse 3.1 nm Ag2Te NCs is shown in the inset of Figure 4b. The thickness of the film is 40 80 nm confirmed by Atomic Force Microscopy (Asylum MFP-3D). Figure 4a shows time-resolved photocurrent with 1 V applied between gold electrodes in the dark and under 750 nm illumination at an intensity of 486 mW/cm2. The distinct photoconductive response supports the inherent semiconducting nature of constituent Ag2Te NCs in the device. The current of the device increases from 2.43 to 4.01 nA under illumination in 25.6 s followed by a slow saturating 4658

dx.doi.org/10.1021/cm2019795 |Chem. Mater. 2011, 23, 4657–4659

Chemistry of Materials behavior. Figure 4b shows a photocurrent decay that can be fit to a biexponential function, I(t) = Aexp( t/t1) + Bexp( t/t2), indicating two major decay processes.34 Two characteristic time constants t1 and t2 represent bulk-like rapid decay process and the slow decay process which is more likely related to surface-traps. The constants A and B are the relative weighting factors of two decaying mechanisms (A + B = 1). Fitting for parameters gives A = 0.428, B = 0.572, t1 = 1.1 s, t2 = 17.2 s. Chemical treatments35,36 allow engineering of the interparticle spacing to modify the mobility of charge carriers, reducing trapping to achieve faster response time,37,38 and controlling doping to manipulate the photocurrent gain.39 More efforts are on the way to obtain the faster response and sensitivity for practical near-infrared optical devices. In the present work, we studied structural, chemical, optical, and optoelectronic properties of Ag2Te NCs. The unique optical properties arise in monodisperse 3.1 nm NCs inconsistent with the bulk crystal structures. We attributed the absorption to an interband transition, as supported by optical absorption measurements as a function of the temperature and the dielectric constant of the environment. The robust spectral responses within the biological transparent window as well as the linear and large temperature dependence create the potential for in vivo temperature marker or energy transducer applications. Photocurrent switching between on and off states provides more support for the semiconducting nature of Ag2Te NCs. The functionality offered from NCs in addition to unusual physical properties of Ag2Te make it a valuable materials system to be explored further.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental section details, X-ray diffraction simulation, and extinction coefficient. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected].

’ ACKNOWLEDGMENT Y.-W.L. and S.J.O.’s respective roles in Ag2Te synthesis and photoconductive studies were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (Award DE-SC0002158) as were CBM and C.R.K.’s supervisory roles. The national science foundation supported D.-K.K. though PENN MRSEC DMR0520020. And T.G. through the Nano/Bio Interface Center at the University of Pennsylvania Grant DMR08-32802. V.D.-N. x-ray modeling was supported by the Department of Energy’s Advanced Research Projects Agency, Energy (ARPA-E) DE-AR0000123. T.P., Y.K., X.Y., and L.J. acknowledge the support of the Office of Naval Research, MURI W911NF-08-1-0364. C.B.M. is grateful for the support of the Richard Perry University Professorship. ’ REFERENCES (1) Sahu, A.; Qi, L.; Kang, M. S.; Deng, D.; Norris, D. J. J. Am. Chem. Soc. 2011, 133, 6509–6512. (2) Yarema, M.; Pichler, S.; Sytnyk, M.; Seyrkammer, R.; Lechner, R. T.; Fritz-Popovski, G.; Jarzab, D.; Szendrei, K.; Resel, R.; Korovyanko, O.; Loi, M. A.; Paris, O.; Hesser, G.; Heiss, W. ACS Nano 2011, 5, 3758–3765.

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