DOI 10.1007/s11082-013-9810-1
Preliminary studies of the Raman spectra
of Ag
2Te and Ag
5Te
3T. I. Milenov · T. Tenev · I. Miloushev · G. V. Avdeev· C. W. Luo · W. C. Chou
Received: 29 April 2013 / Accepted: 12 October 2013 / Published online: 23 October 2013 © Springer Science+Business Media New York 2013
Abstract The theoretical calculations indicated that the monoclinic low-temperature phase of silver telluride(β -Ag2Te) is a new binary topological insulator with highly anisotropic single Dirac cone surface. We obtainedβ -Ag2Te crystal ingots containing few grains by the Bridgman method. We also deposited thin films of tellurium, Ag5Te3and(Te+Ag5Te3) by thermal evaporation method. The Raman spectra ofβ -Ag2Te, tellurium and Ag5Te3 were measured at three excitation wave lengths: 633, 515 and 488 nm. The Raman active modes of
β -Ag2Te, tellurium and Ag5Te3are situated at frequencies below 300 cm−1while vibrations of other phases appear at higher frequencies.
Keywords Semiconductors· Single crystals · Thin films · Raman spectroscopy
1 Introduction
Silver telluride(Ag2Te), one of the silver chalcogenides, is known as the Hessite mineral in nature. It has some unique properties such as superionic conductivity and [as it was recently
T. I. Milenov (
B
)“E. Djakov” Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
e-mail: [email protected] T. Tenev· I. Miloushev
“G. Nadjakov” Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
G. V. Avdeev
“R. Kaishev” Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 11, 1113 Sofia, Bulgaria
C. W. Luo· W. C. Chou
Department of Electrophysics, National Chiao Tung University, No 1001, Ta Hsueh Rd, Hsinchu 300, Taiwan
shown byZhang et al.(2011)] theoretical calculations indicated that the low-temperature phase(β -Ag2Te) is a binary topological insulator with highly anisotropic single Dirac-cone surface. The band-gap values ofβ -Ag2Te reported in literature are inconsistent and vary from 0,01 up to 1.7 eV (Dalven 1966;Prabhune and Fulari 2009). Moreover, the Raman spectrum is also not yet clarified—the interested researcher could easily find information as to what feature in the Raman spectrum of Ag2Te samples does not belong to the Ag2Te vibrations—Qin et al.(2007).
We recently presented a thorough study of the broadband reflectance spectrum of Ag2Te and determined the plasma energy of Ag2Te to be 0.097 eV and the bandgap Eg= 1.44 eV— Hong et al(2013).Lee et al.(2012) established extremely high electron mobility in Ag2Te nanowires (over 22,000 cm2/Vs—see Lee et al. 2012). This makes monoclinic Ag2Te a possible choice for material for FET detectors of THz radiation.
Here we present results on the synthesis of Ag2Te crystals and deposition of thin films of pure Te and Ag5Te3as well as films with mixed phases by thermal evaporation and their study by Raman spectroscopy.
2 Experimental
Ag2Te monoclinic crystals typically containing few grains were grown by the Bridgman method. In order to clarify the Raman spectrum of monoclinic Ag2Te besides the main object of our study(β -Ag2Te) we have to examine specimens with different compositions: pure tellurium, Ag5Te3(as some residual quantities of Ag5Te3could be found in Ag2Te ingots) and two-phase films consisting of tellurium and Ag5Te3 (by analogy). This problem was simply solved by deposition of thin films by thermal evaporation technique and by varying the deposition rate and time thin films of different compositions were obtained. The thickness of deposited films is controlled during the process by a build-in detector. The exact values of the deposition rate and films’ thickness are summarized in Table1.
The qualitative analysis of the Ag2Te crystals and the thin films was performed by X-ray powder diffraction in Philips PW1050 diffractometer, equipped with a Cu Kαtube. In order to determine the exact orientation of surface of the specimen for Raman spectroscopy we examined each of the examined surfaces by XRD on the same diffractometer. The data were collected inθ–2θ step-scan mode in the angle interval from 20◦to 50◦(2θ) at steps of 0.05◦(2θ) and counting time of 3 s/step. The phase homogeneity and composition of
the deposited films was determined by a SEM/EDX study in a LYRA/TESCAN scanning electron microscope equipped with Brucker Quantax 400 unit for EDX measurements.
Table 1 Results of phase homogeneity and composition studies of polycrystalline Ag2Te and thin film specimens
Specimen Ag2Te ingot Thin films Film thickness/rate
of deposition
10 nm/0.5 Å/s 350 nm/1.2 Å/s 600 nm/ 0.9 Å/s Phases Single phase Single phase Single phase Two phases
Composition 100 % Ag2Te Pure Te Ag5Te3 Dark phase denoted as PEAK in Fig.2: (65 % Ag5Te3+35 % Te) Bright phase denoted as FLAT in Fig.2: Ag5Te3
Raman spectra were measured using HORIBA Jobin Yvon Labram and an Ar+ laser (for 515 and 488 nm excitation wavelengths) and a He–Ne laser for 633 nm excitation. The irradiated spot and the absolute accuracy were 5μm and 0.5 cm−1, respectively. The laser power was limited to 300μW in order to prevent damages of the studied specimens in most of measurements. Some Raman spectra were excited at higher laser power to investigate irradiation-induced changes in the samples.
3 Results and discussion 3.1 XRD and SEM
The XRD powder pattern (Fig.1upper trace) coincides with the data published bySchneider and Schulz(1993) for monoclinic Ag2Te. The XRD pattern of the plane perpendicular to the growth axis of a Ag2Te crystal grown by the Bridgman method implies the conclusion that the irradiated area is single crystalline and is parallel to the(212) plane—see Fig.1lower trace.
The phase homogeneity and composition are studied by powder XRD analysis and SEM/EDX observations and measurements. It is established that only specimens of about 600 nm thick layers consist of two phases—see Fig.2. The brighter phase, denoted as FLAT in Fig.2has composition of pure Ag5Te3while the darker one, denoted as “PEAK” in Fig.2 has composition of (65 % Ag5Te3+ 35 % Te). The results of XRD and SEM/EDX study are summarized in Table1.
3.2 Raman spectrum of Ag2Te
Our experience shows that Ag2Te is very soft material and we have not yet found a suit-able etching agent for its chemical polishing. For this reason we used for Raman spec-troscopic measurements the as grown (212)plane, which is the plane perpendicular to
Fig. 1 Powder X-ray diffraction (XRD) pattern of Ag2Te- upper trace and XRD of the(212)plane (perpen-dicular to the growth axis)—lower trace
Fig. 2 The back-scattered electron SEM image of 600 nm thick layer. The bright phase marked by “FLAT” in
the image consists of pure Ag5Te3. The darker one, denoted as “PEAK” has composition of (65 % Ag5Te3+ 35 % Te)
Fig. 3 Unpolarized and (XX) and (XY) polarized Raman spectra of Ag2Te. The magnified low-frequency part of the polarized spectra is shown in the inset
the growth axis. Prior to the measurements the specimen was ultrasonically cleaned ini-tially in ethanol and further in acetone. The polarized Raman measurements were car-ried out in back-scattering geometry in parallel (XX) and perpendicular (crossed; XY) configuration.
The full vibrational representation of Ag2Te at k= 0 contains = 3Ag+3Bg+3Au+3Bu modes (space group P 21/c (No. 14), unique axis b and point group C2h(2/m)). The Agand Bg modes are Raman active, while Auand Bumodes are IR active (seeAroyo et al. 2006a,b).
The Raman spectrum of polycrystalline Ag2Te sample is shown in the Fig.3—upper trace. Three broad bands with full width at a half maximum (FWHM) of about 18–20 cm−1 and centered at about 80, 110 and 138 cm−1 are distinguishable. The high values of the FWHM tends to suspicion that these features consists of more than one mode. We conduct
Fig. 4 a XX-polarized Raman spectra of 10 nm thick pure tellurium layer excited at 633, 515 and 488 nm
laser wavelengths; b same as (a) in XY polarization—see text
additional measurements of the polarized Raman spectra ofβ -Ag2Te single crystal. The notch filter cuts the wavelengths below 90 cm−1and therefore the band centered at about 80 cm−1is impossible to be studied further. Then we suggest that the band situated at about 138 cm−1includes two modes: marked by 3 at 131 and marked by 4 at 140 cm−1. Another band seems also complex and two modes (marked by 1 at 101 cm−1and marked by 2 at 108 cm−1) are supposed to form the feature centered at about 110 cm−1. Moreover, the intensity ratio I2/I4of the modes 2 and 4 remains unchanged while the intensity ratio I4/I3 significantly drops upon change of the scattering geometry from (XX) to (XY) scattering geometry. We therefore tentatively ascribe line 3 to a Bg−and line 4 to an Ag− vibration. We do not observe any other feature in the higher frequency part of the Raman spectrum of Ag2Te crystals.
Fig. 5 (XX) and (XY) polarized Raman spectra of about 600 nm thick layer containing(Ag5Te3) and (65 % Ag5Te3+ 35 % Te) phases
3.3 Raman spectrum of pure tellurium and Ag5Te3
According to the binary phase diagram Ag–Te (see for exampleKarakaya and Thompson (1991)) there are only three phases stable at room temperature: Ag2Te, Ag5Te3 and Te. In this sense, the Raman spectrum of Te and Ag5Te3 should also be measured in order to accurately establish the spectrum of Ag2Te. We observed two clearly distinguishable modes in the Raman spectrum of pure tellurium—Fig.4a and b: the Ag mode at 121 cm−1and Egmode at 141.9 cm−1. These modes practically coincide with those described byPine and Dresselhause(1971)—120.4 and 140.7 cm−1, respectively. The frequency of these vibrations does not depend on the wavelength of the exciting laser light.
The Raman spectrum of Ag5Te3in the frequency range 90–1,200 cm−1(see Fig.5) con-tains a broad feature at about 150 cm−1and a weak feature at about 159 cm−1, distinguishable in parallel scattering geometry.
3.4 Raman spectra of Ag2Te specimens damaged by the laser beam
We also performed Raman measurements on Ag2Te single crystal specimens irradiated with laser power higher than 0.5 mW in order to evaluate the Raman spectrum of phases decom-posing due to laser-induced heating. We present in Fig.6the Raman spectra of specimens irradiated with laser power of 0.7 mW at 633 nm (upper trace) and with 20 and 40 mW at 515 nm (middle trace and lower trace, respectively). We focused the laser beam on a spot of diameter of 5μm in all measurements in order to retard the decomposition process.
It is clearly distinguishable that too strong laser-beam irradiation causes an Ag2Te phase decomposition according to the following the redox reaction (Qin et al. 2007):
Ag2Te+ O2→ 2Ag + TeO2, (1)
The Raman spectra of the specimens exposed to higher laser power consist of the spectra of different polymorphic forms of tellurium oxide(Te2O). The spectrum measured at lower laser power (0.6 mW at 633 nm wavelength) (see the BR1-marked upper trace in Fig.6) is
Fig. 6 Unpolarized Raman spectra of Ag2Te single crystal specimens excited at 633 nm with 0.6 mW laser power (BR1-marked upper trace), at 515 nm with 20 mW laser power-BR2 marked middle trace and at 515 mW with 40 mW laser power-BR3 marked lower trace
dominated by vibrations of the metastableγ -TeO2−seeChamparnaud-Mesjard et al.(2000): A1and B1modes at 115, 140, 173, 417, 608 and 680, and 816 cm−1, respectively. The stable
α- and β- polymorphic forms of TeO2(Kondratyuk et al. 1987andBeyer 1967) are achieved by increasing the laser power to 20 and 40 mW at 515 nm wavelength, respectively.
The spectrum excited with the highest laser power (BR3-marked lower trace in Fig.6) could be attributed to a mix ofα- and γ -TeO2dominated byα -TeO2(Champarnaud-Mesjard et al. 2000andMirgorodsky et al. 2000) and contains A1, A2and E-modes ofα - TeO2at: 390, 643 and 379, 642 and 225 cm−1, respectively. The shoulders at about 180 cm−1and at about 672 cm−1(not marked in Fig.6) could be ascribed to coinciding B1gand B3gand Ag modes ofβ-polymorphic forms of TeO2−Mirgorodsky et al.(2000).
The spectrum excited with 20 mW (515 nm excitation wavelength) contains simultane-ously modes ofα-, β- and γ -TeO2−see Fig.6—middle trace.
4 Conclusions
We successfully synthesized crystals of the monoclinic (β-phase) of Ag2Te and deposited thin films of Te and Ag5Te3and of mixed phases [Ag5Te3+ (65% Ag5Te3+ 35% Te)]. The obtained Raman response from the crystals and thin films enables us to conclude that the Raman spectrum (in the spectral range 70–600 cm−1) of β -Ag2Te consists of two bands at 111 and 134 cm−1and a broad feature at about 80 cm−1. We suggest that the bands arise due to merging of Bgand Ag modes at 101 and 108 cm−1and most probably 2 Agmodes at 131 and 141 cm−1, respectively. The Raman spectrum of Ag5Te3 in the same frequency range contains a broad feature at about 151 cm−1and a weak shoulder at 159 cm−1, while that of pure tellurium exhibits an Agmode at 121 cm−1and an Egmode at 141 cm−1. We establish that excitation by a laser power higher than 350μW decomposes Ag2Te to silver and different polymorphic forms of tellurium oxide TeO2.
Acknowledgments The authors gratefully acknowledge Prof. M. Abrashev (Faculty of Physics, Sofia Uni-versity, Sofia) for help with the Raman measurements. This work is financially sponsored by National Science
Council of Taiwan (Grant No. NSC 98-2112-M-009-008-MY3) and is performed in the framework of COST Action MP 1204 of the EU.
References
Aroyo, M.I., Kirov, A., Capillas, C., Perez-Mato, J.M., Wondratschek, H.: Bilbao crystallographic server II: representations of crystallographic point groups and space groups. Acta Crystallogr. A 62, 115–128 (2006a) Aroyo, M.I., Perez-Mato, J.M., Capillas, C., Kroumova, E., Ivantchev, S., Madariaga, G., Kirov, A., Won-dratschek, H.: Bilbao crystallographic server I: databases and crystallographic computing programs. Z. Kristallogr. 221, 15–27 (2006b)
Beyer, V.H.: Verfeinerung der Kristallstruktur von Tellurit, dem rhombischen TeO2. Z. Kristallogr. 124, 228 (1967)
Champarnaud-Mesjard, J.C., Blanchandin, S., Thomas, P., Mirgorodsky, A., Merle-Méjean, T., Frit, B.: Crystal structure, Raman spectrum and lattice dynamics of a new metastableform of tellurium dioxide: gamma-TeO2. J. Phys. Chem. Sol. 61, 1499–1507 (2000)
Dalven, R.: Fundamental optical absorption inβ-silver telluride. Phys. Rev. Lett. 16, 311–312 (1966) Hong, C.-C., Luo, C.-W., Milenov, T.I.: Reflectance spectroscopy studies of Ag2Te Crystals. In: Annual
Meeting of the Physical Society of Taiwan, Annual Meeting of the Physical Society of Taiwan, National Dong Hwa University, Hualien, Republic of China (2013)
Karakaya, I., Thompson, W.T.: The Ag–Te (Silver–Tellurium) system. J. Phase Equilib. 12, 57–63 (1991) Kondratyuk, I.P., Muradyan, L.A., Pisarevsky, I.V., Simonov, V.I.: Precision X-ray structural investigation of
acousto-optical single crystals ofα-TeO2, Kristallografiya 32, 609 (1987), in Russian
Lee, S., In, J., Yoo, Y., Jo, Y., Park, Y.C., Kim, H.J., Koo, H.C., Kim, J., Kim, B., Wang, K.L.: Single crystalline
β-Ag2Te nanowire as a new topological insulator. Nano Lett. 12, 4194–4199 (2012)
Mirgorodsky, A.P., Merle-Méjean, T., Champarnaud, J.-C., Thomas, P., Frit, B.: Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new
γ -andδ-phases. J. Phys. Chem. Sol. 61, 501–509 (2000)
Pine, A.S., Dresselhause, G.: Raman scattering in paratellurite, TeO2. Phys. Rev. B 4, 356 (1971)
Prabhune, V.B., Fulari, V.J.: Measurement of properties of silver telluride thin film using holography. Opt. Commun. 282, 2118–2122 (2009)
Qin, A., Fang, Y., Tao, P., Zhang, J., Su, C.: Silver telluride nanotubes prepared by the hydrothermal method. Inorg. Chem. 46, 7403–7409 (2007)
Schneider, J., Schulz, H.: X-ray powder diffraction of Ag2Te at temperatures up to 1123 K. Z. Kristallogr.
203, 1–15 (1993)
Zhang, W., Yu, R., Feng, W., Yao, Y., Weng, H., Dai, X., Fang, Z.: Topological aspect and quantum magne-toresistance ofβ-Ag2Te. Phys. Rev. Lett. 106, 156808 (2011)
