C.H. Lianga, Y.H. Chana, K.K. Tionga,∗, Y.S. Huangb, Y.M. Chenb, D.O. Dumcencob, C.H. Hoc
aDepartment of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
bDepartment of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
cDepartment of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
a r t i c l e i n f o
Article history:
Received 26 June 2008 Received in revised form 25 September 2008 Accepted 26 September 2008 Available online xxx
PACS:
71.20.Be 81.10.−h 68.37.Og Keywords:
Transition-metal alloys and compounds Crystal growth
Anisotropy TEM
a b s t r a c t
Au-doped rhenium disulfide (Au-ReS2) layer crystals were grown by chemical vapour method with iodine as the transporting agent. The thin samples were investigated by polarization-dependent transmittance (T) and photoreflectance (PR) at 20 K with the optical polarization along and perpendicular to the crys-tal b-axis. The Au-ReS2showed band gap anisotropy with respect to the b-axis. By comparing with the undoped sample, the indirect band gap deduced from the polarization-dependent T spectra shows a more pronounced red shift than the band edge excitonic transitions determined from polarization-dependent PR spectra.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Transition-metal dichalcogenides (TMDC) crystals (MX2) con-sist of stacks of hexagonal close-packed layers of transition-metal atoms (M) sandwiched between two layers of chalcogen atoms (X = S, Se)[1,2]. TMDCs crystallize with strong intralayer covalent bonds and weak van der Waals-type interlayer bonding forces.
ReS2crystallizes in a distorted CdCl2structure of triclinic symme-try (space group P ¯1)[3,4], with clustering of Re4 diamond units forming a one-dimensional chain within the van der Waals plane.
ReS2semiconductors are of interest because of their potential for application in solar cell material for electrochemical cells[5–7]. The transporting agents are an important factor in determining the type of majority carrier in the crystal produced. For instance, transport of ReS2with bromine produces n-type crystals, while iodine pro-duces p-type crystals[3]. In this article, we report on the growth and optical characterization of Au-ReS2crystals prepared by chem-ical vapour transport method with iodine as a transporting agent.
Hall effects measurements confirmed the p-type semiconducting behaviour of the synthesized samples. The crystals formed thin
∗ Corresponding author. Tel.: +886 2 24622192x6208; fax: +886 2 24635408.
E-mail address:[email protected](K.K. Tiong).
layer plates with a typical surface area of 8 mm× 8 mm and thick-ness of a few microns. The strong anisotropy in the chemical bonds leads to anisotropy of the optical and electrical properties of these materials parallel and perpendicular to the b-axis[8]. The opti-cal anisotropic properties were studied by polarization-dependent transmittance (T) measurements parallel and perpendicular to b-axis at 20 K, which yield information for determining the indirect band gap of the semiconductor. The polarization-dependent pho-toreflectance (PR) spectra were used for the determination of the excitonic transitions. The effects of Au dopant on the anisotropic optical properties were studied and discussed.
2. Experimental
Au-doped ReS2layer compounds were prepared by vapour transport methods.
The total charge used in each growth experiment was about 10 g. For crystal growth, the elements (Au, 99.999%; Re, 99.99%; S, 99.999%) was placed in a clean quartz ampoule (22 mm OD, 17 mm ID, 20 cm length) with I2(∼10 mg/cm3) as the transport agent, evacuated to 10−5Torr and sealed. The ampoule was placed in a three-zone furnace. The temperature gradient of about 3◦C/cm with the temperature range from 1050 to 990◦C over a reaction length of 20 cm gives optimal condition for the crystallization of the samples. This gradient was maintained for 4 weeks to com-plete the transported growth of sample from high to low temperature zones and allowed to cool down slowly (40◦C/h) to room temperature. When the ampoule reached room temperature, it was opened and the crystals removed. The XRD pat-tern (peak positions and intensities) of Au-ReS2crystals is similar to that of the undoped ReS2[2]. The details of the optical diagnostic techniques of polarization-0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.175
Please cite this article in press as: C.H. Liang, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.09.175
2 C.H. Liang et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx
Fig. 1. TEM detailed image of the atomic arrangement and its fast Fourier transform (FFT) diffraction is shown in inset.
dependent transmittance and photoreflectance measurements have been described elsewhere[9].
3. Results and discussion
The photograph displayed inFig. 1is the transmission electronic microscope (TEM) image of the crystal morphology for Au-ReS2 while the inset inFig. 1showed the fast Fourier transform (FFT) of the TEM image. TEM investigations were done employing a Philips Tecnai G2 F20 using an acceleration voltage Uacc= 200 kV. The b-axis corresponds to the longest edge of the plate and is parallel to the Re cluster chains[8]. From FFT, the orientation of the b-axis is determined to be along the [0 1 0] direction. Mechanically, one can
also determine b-axis direction to be along the edge of easy cleaving of the crystal plate.
The indirect band-edge transitions of the Au-ReS2were studied by means of near-normal incident polarization-dependent trans-mittance at 20 K. The experimentally measured transtrans-mittance data were utilized to derive the absorption coefficient ˛ (depicted in Fig. 2) using the equation[10].
T =IT
I0 = (1− R)2exp(−˛d)
1− R2exp(−2˛d) (1)
In performing the calculation, we assumed that the temperature dependence of reflectivity R is negligible. In Eq.(1), d is the thick-ness, ITis the transmitted intensity, and I0is the incident intensity.
Also displayed inFig. 2is a plot of (˛h)1/2vs. h, where the open circles (open squares) are deduced data points from the E||b (E⊥b) polarization measurements and the solid lines are least-squares fits to the expression[10].
˛h =A(h − Egind+ Ep)2
exp(−Ep/kT) − 1 +B(h − Eindg − Ep)2
1− exp(−Ep/kT) (2)
The fitted values of the energy gaps and phonon energy of the Au-ReS2 crystals at 20 K are Egind = 1.49 ± 0.02 eV, Eindg⊥ = 1.51 ± 0.02 eV, and Ep= 20± 5 meV. The error for the estimation of Egind , Eindg⊥, and Epwere derived by considering different range of data points in the estimation and also from the slight deviation of inci-dent angles due to non-smooth sample surface. Here, Eindg and Eg⊥ind refer, respectively, to the indirect gap for E||b and E⊥b polarizations.
Analysis of the absorption spectra revealed band gap anisotropy with Eindg⊥ > Eindg and their values are red shifted by about 10 meV comparing to that of the pure samples[9,11]. The physical origin of the shift may come from the existence of impurity, which in general will contribute to the absorption near the band tail.
Displayed by the curves inFig. 3are the polarization-dependent PR spectra in the vicinity of the direct gap of Au-ReS2at 20 K. The solid lines inFig. 3are least-squares fits to the derivative Lorentzian line shape[12], which yield parameters of the transition energies
Fig. 2. Polarization-dependent absorption coefficient ˛ vs. h and (˛h)1/2vs. h for Au-ReS2at 20 K.
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C.H. Liang et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx 3
Fig. 3. The polarization-dependent PR spectra of Au-ReS2at 20 K.
indicated by arrows.
R
R = Re[n
i=1Aexi ejϕexi (E − Eiex+ jiex)−2] (3) In Eq.(3) the subscript i refers to the type of interband transi-tion, Eiexand iexare the energy and broadening parameter of the interband excitonic transition, Aexi and ϕexi are the amplitude and phase of the line shape, respectively, and the value of n depends on the origin of the transition. For the first derivative functional form, n = 2.0 is appropriate for bounded states such as excitons or impurity transitions[12]. Arrows at the bottom of PR spectra show the peak positions of the main excitonic features, with Eex1 = 1.554 ± 0.002 eV, Eex2 = 1.585 ± 0.002 eV, Eex3(Eex3⊥) = 1.642± 0.002 (1.645± 0.002) eV, and Eex4(E4⊥ex) = 1.656± 0.002 (1.659 ± 0.002) eV.
Comparing with the undoped sample[9], our results showed a small red shift of 1–2 meV for the band edge excitonic transitions.
The Eex1 , E3ex and Eex4 features were detected for E||b polarization while Eex2 , E3⊥exand Eex4⊥features were observed for E⊥b polarization only[9]. The different polarization dependence of these observed excitonic transitions of ReS2lends evidence that Eex1 , E2ex, and Eex3
(Eex3⊥)/E4ex) (E4ex⊥) are related to the interband excitonic transitions of different origins[9].
4. Summary
We have demonstrated the synthesis of single crystals of Au-doped ReS2with surfaces up to 8 mm× 8 mm and few micrometers thickness by chemical vapour transport method using iodine as a transporting agent. The indirect band gap and that of the direct band edge excitonic transition energies were deduced from the low temperature polarization-dependent T and PR measurements.
The Au-doped ReS2 showed both indirect and direct band gap anisotropy along and perpendicular to the crystal b-axis. Our mea-surements indicate that gold can incorporate into the host lattice ReS2 and the effect of red shift on the indirect band gap is more pronounced than that of the band edge excitonic transitions.
Acknowledgements
The authors acknowledge the support of the National Science Council of Taiwan under the projects nos. NSC 96-2112-M-019-003 and NSC 96-2112-M-011-001.
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