Crystal growth characterization and electronic band structure of TiSeS Y. Shemerliuk1 A. Kuibarov1 O. Feia13 M. Behnami1 H. Reichlova12 O. Suvorov1 S.

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Crystal growth, characterization and electronic band structure

of TiSeS

Y. Shemerliuk1, A. Kuibarov1, O. Feia1,3, M. Behnami1, H. Reichlova1&2, O. Suvorov1, S.

Selter1, D.V. Efremov1, S. Borisenko1, B. Büchner1&2, S. Aswartham1

1 Institut für Festkörperforschung, Leibniz IFW Dresden, Helmholtzstraße 20, 01069 Dresden,

Germany

2 Institut für Festkörper- und Materialphysik and Würzburg-Dresden Cluster of Excellence

ct.qmat, Technische Universität Dresden, 01062 Dresden, Germany

3Kyiv Academic University, 03142, Kyiv, Ukraine

Abstract

Layered semimetallic van der Waals materials TiSe2 has attracted a lot of attention

because of interplay of a charge density wave (CDW) state and superconductivity. Its sister

compound TiS2, being isovalent to TiSe2 and having the same crystal structure, shows a

semiconducting behavior. The natural question rises - what happens at the transition point in

TiSe2-xSx, which is expected for x close to 1. Here we report the growth and characterization of

TiSeS single crystals and the study of the electronic structure using density functional theory

(DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have

the same morphology as TiSe2. Transport measurements reveal a metallic state, no evidence of

CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar

to that of TiSe2, but the electron and hole pockets in TiSeS are much smaller. The ARPES

results are in good agreement with the calculations.

Keywords: Crystal growth; chemical vapor transport; 2D- van der Waals crystals; XRD, CDW,

DFT, ARPES

1. Introduction

In recent years, research on functional two-dimensional (2D) materials has stimulated

activities aimed at synthesizing new materiaAls with novel functional properties. Due to their

unique electronic [1-3], magnetic [4-6], and optical properties [7-9], the new 2D materials are

expected to have great potential for applications. One of the most interesting class is the

transition metal dichalcogenides MX2 (M = transition metal, X = chalcogenide). They and their

mixed systems dominate among the current 2D materials due to their favorable structural [10,

11], optical, electronic behavior [12-16].

MX2 could be stabilized in a trigonal 1T, a hexagonal 2H, or a rhombohedral 3R structural

phase. The electronic properties of MX2 range from semiconductors [17] to semimetals [18, 19-

20], from material with a trivial electronic structure till topological semimetals [21] and

topological insulators [22]. Due to these unique properties, MX2 continue to attract the scientific

community.

TiSeS is a member of the trigonal 2D MX2 family with the space group of P3 ̅m1(No.

164), where Ti atoms are sandwiched between two layers of S and Se atoms and each TiSeS

layer is bound by the van der Waals (vdW) interaction. Therefore, the individual layers can be

easily exfoliated. One of two pristine compounds of the mixed crystal system TiSe2-xSx (0 ≤ x

≤ 2) considered with x = 2 is a semiconductor with an indirect band gap, while another with x

= 0 is a semimetal with slightly overlapping bands [23]. In TiSe2 compounds, a charge density

wave (CDW) transition is observed at about 200 K at ambient pressure [24, 25]. TiSe2 grown

using high pressure shows insulating behavior at low temperatures [26]. However, Cu-

intercalation or application of hydrostatic pressure suppress the CDW state, giving room for

superconductivity [27, 28]. It was shown that the maximum superconducting Tc corresponds to

the CDW quantum critical point [29].

Motivated by this nontrivial behavior, in our work we try to use chemical pressure by

substitution of Se by S to tune the material to the CDW quantum critical point. Since the ionic

radius of S is smaller than Se, the substitution acts as an external pressure. As the first step, we

optimized the synthesis and crystal growth conditions of TiSeS.. Further, we have investigated

the crystal structure, magnetic and transport properties of as grown single crystals. Finally, we

performed the band structure calculations using DFT and compare it with the ARPES

measurements.

2. Experimental methods

The crystals were obtained from the Chemical Vapor Transport (CVT) crystal growth

experiment. The chemical composition of our crystals was investigated using X-ray energy

dispersive x-ray spectroscopy (EDX), with an accelerating voltage of 30 kV. Electron

microscopic images were obtained by using a scanning electron microscope with two types of

signals: the secondary electrons (SE) for topographic contrast and the backscattered electrons

(BSE) for chemical contrast.

The crystal structure was investigated by powder X-ray diffraction (pXRD) using a STOE

powder laboratory diffractometer in transmission geometry with Cu-Kα1 radiation (the

wavelength (𝜆) is 1.540560 Å) from a curved Ge (111) single crystal monochromator and

detected by a MYTHEN 1K 12.5◦ linear position sensitive detector manufactured by DECTRIS.

An XRD pattern of a polycrystalline sample was obtained by grinding as-grown single crystals.

Temperature and field dependent magnetization were measured on bulk as-grown crystal using

a Quantum Design Superconducting Quantum Interference Device Vibrating Sample

Magnetometer (SQUID-VSM). Transport measurements were carried on single crystals and the

wires were glued by a conductive two component epoxy. Four contacts at the top surface (ab-

plane) and two contacts on the bottom surface of the sample were prepared to allow for four

probe method in both [in plane direction] and [c direction]. The measurements were performed

in Oxford 15 T cryostat and the temperature was controlled by a heater at the sample holder.

The electric current was applied either in the [ab plane] or [c direction], longitudinal and

transversal voltage was measured. Magnetic field was applied perpendicular to the ab-plane for

the charge carrier density measurements from Hall effect, the transversal data were

antisymmetrized prior the mobility evaluation. Several samples were prepared showing similar

results. ARPES measurements of TiSeS were performed on 12 station at BESSY synchrotron

with Scienta Omicron R8000 energy analyzer with total energy and momentum resolution less

than 10 meV and 0.01 Å-1 respectively. High quality single crystals were cleaved and measured

in a chamber with pressure better than 8*10-11 mbar at a temperature less than 15 K.

Crystal Growth via Chemical Vapor Transport: TiSeS single crystals were grown by the

chemical vapor transport technique. All preparation steps were performed under argon

atmosphere in a glovebox, before sealing the ampule. The starting materials titanium (powder,

Alfa Aesar, 99,99%) selenium (pieces, Alfa Aesar, 99.97%) and sulfur (pieces, Alfa Aesar,

99.999%) were weighed out with a molar ratio of Ti:Se:S = 1:1:1 and homogenized in an agate

mortar. 1g of the starting material was loaded in a quartz ampoule (10mm inner diameter, 3mm

wall thickness) together with 0.04 g of the transport agent iodine. The filled ampoule was cooled

by liquid nitrogen, evacuated to a residual pressure of 10−8 bar and sealed at a length of

approximately 12 cm by the oxy-hydrogen flame under static pressure.

The closed ampule was heated in a two-zone furnace with the following temperature

profile optimized by us. Initially, the furnace was heated homogeneously to 7900C with

1000C/h. After that, an inverse transport gradient is applied to transport particles to the one side

of the ampoule which is the charge region. This region was kept at 7900C for 336 h. The other

side of the ampoule which is the sink region was initially heated up to 8400C at 1000C/h, and

dwelled at this temperature for 24 h. After that, the temperature in the growth zone was

gradually reduced during one day to 7300C to slowly form the transport temperature gradient

for controlling nucleation and held at this temperature for 402 h. As a result, the temperature

gradient was set for vapor transport between 7900C (charge) and 7300C (sink) for 17 days.

Finally, the charge region was cooled to the sink temperature in 2 hours before both regions

were furnace-cooled to room temperature.

Thin lustrous plate-like crystals of TiSeS perpendicular to the c-axis in the size of

approximately 3 mm × 3 mm × 100 μm were obtained. As example, as-grown single crystals

are shown in Fig. 1(a-b). All of these crystals show a layered morphology and they are easily

exfoliated by scotch tape.

Figure 1 (a) and (b): As-grown crystals of TiSeS by the chemical vapor transport, cell scale

is 1 mm

(a)

(b)

Characterization: compositional and structural analysis

As-grown single crystals exhibit the typical features of layered systems, such as steps and

terraces, as shown in Fig. 1(a-b). The topographical SE image of TiSeS crystal has a well-

defined flat hexagonal facade with angles of 120°, which clearly indicates that they grew along

the symmetry axes (Fig 2(a)). Back scattered electron (BSE) image of our crystal has

homogeneous chemical contrast over the surface of the crystal, as shown in Fig. 2(b). This

indicates a homogeneous elemental composition on the respective area of the crystal. At some

small areas, the observed contrast changes can be clearly attributed to some scratches on top of

the crystal and not to compositional changes, as clearly seen by comparing with the SE image.

The elemental composition of the TiSeS single crystals was determined by energy-

dispersive x-ray spectroscopy (EDX) via measuring different areas and points on the surface of

the crystal. The compositional analysis of as-grown single crystals is Ti34.4(1)Se30.2(9)S35.3(2). The

compound shows the expected composition within the error of this measurement technique.

The result of EDX measurements highly depends on the sample topography and the error

regarding each element corresponds to an order of up to 5 at% [30].

The structural characterization and phase purity were confirmed by powder x-ray

diffraction using a STOE powder diffractometer. The pXRD pattern obtained from TiSeS

crystals, as shown in Fig. 3, was indexed in the space group P3

̅m1(No. 164), in agreement with

literature [31]. No additional reflections were observed demonstrating the phase purity of our

crystals. Starting from the crystal structure model proposed by Bozorth [32], a refined crystal

structure model is obtained using the Rietveld method. Figure 3 together with the calculated

Figure 2. SEM image of an as-grown TiSeS crystal with topographical contrast (SE mode) in

(a) and chemical contrast (BSE mode) in (b).

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Crystalgrowth,characterizationandelectronicbandstructureofTiSeSY.Shemerliuk1,A.Kuibarov1,O.Feia1,3,M.Behnami1,H.Reichlova1&2,O.Suvorov1,S.Selter1,D.V.Efremov1,S.Borisenko1,B.Büchner1&2,S.Aswartham11InstitutfürFestkörperforschung,LeibnizIFWDresden,Helmholtzstraße20,01069Dresden,Germany2InstitutfürFes...

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Crystal growth characterization and electronic band structure of TiSeS Y. Shemerliuk1 A. Kuibarov1 O. Feia13 M. Behnami1 H. Reichlova12 O. Suvorov1 S.

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