Stoichiometry dependence of the transport properties of TiS2

Probing the photocurrent in two-dimensional titanium disulfide

Generating photocurrent in a condensed matter system involves the excitation, relaxation, and transportation of charge carriers. As such, it is viewed a potent method for probing the dynamics of non-equilibrium carriers and the electronic band structure of solid state materials. In this research, we analyze the photoresponse of the mechanically exfoliated titanium disulfide (TiS2), a transition metal dichalcogenide whose classification as either a semimetal or a semiconductor has been the subject of debate for years. The scanning photocurrent microscopy and the temperature-dependent photoresponse characterization expose the appearance of a photovoltaic current primarily from the metal/TiS2junction in an unbiased sample, while negative photoconductivity due to the bolometric effect is observed in the conductive TiS2channel. The optoelectronic experimental results, combined with electrical transport characterization and angle-resolved photoemission spectroscopy measurements, indicate that the TiS2employed in this study is likely a heavily-doped semiconductor. Our findings unveil the photocurrent generation mechanism of two dimensional TiS2, highlighting its prospective optoelectronic applications in the future.

High-performance flexible thermoelectric modules based on high crystal quality printed TiS2/hexylamine

Printed electronics implies the use of low-cost, scalable, printing technologies to fabricate electronic devices and circuits on flexible substrates, such as paper or plastics. The development of this new electronic is currently expanding because of the emergence of the internet-of-everything. Although lot of attention has been paid to functional inks based on organic semiconductors, another class of inks is based on nanoparticles obtained from exfoliated 2D materials, such as graphene and metal sulfides. The ultimate scientific and technological challenge is to find a strategy where the exfoliated nanoparticle flakes in the inks can, after solvent evaporation, form a solid which displays performances equal to the single crystal of the 2D material. In this context, a printed layer, formed from an ink composed of nano-flakes of TiS₂ intercalated with hexylamine, which displays thermoelectric properties superior to organic intercalated TiS₂ single crystals, is demonstrated for the first time. The choice of the fraction of exfoliated nano-flakes appears to be a key to the forming of a new self-organized layered material by solvent evaporation. The printed layer is an efficient n-type thermoelectric material which complements the p-type printable organic semiconductors The thermoelectric power factor of the printed TiS₂/hexylamine thin films reach record values of 1460 µW m^-1 K^-2 at 430 K, this is considerably higher than the high value of 900 µW m^-1 K^-2 at 300 K reported for a single crystal. A printed thermoelectric generator based on eight legs of TiS₂ confirms the high-power factor values by generating a power density of 16.0 W m^-2 at ΔT = 40 K.

Effect of the Titanium Self-Intercalation on the Electronic Structure of TiSe2

The effect of the superstoichiometric Ti intercalation on the electronic structure of TiSe₂ was studied by using X-ray photoelectron spectroscopy in nonresonant and resonant modes along with the DOS (density of states) calculations. It was shown that the presence of the Ti atoms in the interlayer space leads to the formation of the Ti 3d_z²/Ti 3d_z² hybridized band between the Ti atoms in the regular lattice positions and Ti atoms in the interlayer space. The charge transfer to the conduction band was not observed in this case.

Direct femtosecond laser ablation of large-area TaSe2, SnS2, and TiS2 thick films by a back ablation procedure

Direct ablation of large-area graphene-like two-dimensional (2D) materials, i.e., tantalum diselenide (TaSe₂), stannic disulfide (SnS₂), and titanium disulfide (TiS₂), by the back ablation method with a femtosecond laser with a repetition rate of 50 MHz and pulse width of 200 fs is studied for the first time to our knowledge. The ablation thresholds of the three kinds of materials are discussed. In addition, the optimization and ablation of narrow grooves on the films are demonstrated. Our work demonstrates the direct femtosecond laser ablation processing of the graphene-like 2D-material films and the potential of 2D-material-film-based devices.

Thermoelectric Properties of (Ba,K)Zn2As2 Crystallized in the ThCr2Si2-type Structure

The compound Ba_1-xK_xZn₂As₂ has a low-temperature phase (α-phase) crystallized in the α-BaCu₂S₂-type structure and a high-temperature phase (β-phase) crystallized in the ThCr₂Si₂-type structure. We successfully obtained the β-phase at room temperature as a metastable state by quenching from above the structural phase transition. This allowed us to determine the thermoelectric properties of the β-phase from room to high temperature in the range of 0.00 ≤ x ≤ 0.10. The lattice thermal conductivity is quite low, with a value less than 1 W/mK at 773 K, independent of x. The effective suppression may be due to lattice instability in the underdoped region and to randomness in the overdoped region. The maximum dimensionless figure-of-merit ZT was 0.30 at 773 K for x = 0.03 with the power factor of 0.61 mW/mK², which is relatively high for a ThCr₂Si₂-type structure. The results demonstrate the effectiveness of quenching for obtaining a low lattice thermal conductivity, thus providing a new method for attaining high thermoelectric performance.

Texturization-Induced In-Plane High-Performance Thermoelectrics and Inapplicability of the Debye Model to Out-of-Plane Lattice Thermal Conductivity in Misfit-Layered Chalcogenides

Texturization tuning is of crucial significance for designing and developing high-performance thermoelectric materials and devices. Here, we report for the first time that a strong texturization effect induces an in-plane high-performance thermoelectric and an out-of-plane low lattice thermal conductivity in Sb-substituted misfit-layered (SnS)_1.2(TiS₂)₂ alloys. In the in-plane direction, the oriented texture promotes a high carrier mobility, contributing to the maximization of the power factor (∼0.90 mW K^-2 m^-1). Moreover, the in-plane lattice thermal conductivity dramatically reduces deriving from the point defects due to the Sb substitution and weakened transverse sound velocity owing to the softening of bonding, ultimately leading to one of the highest thermoelectric performances ever reported among misfit-layered chalcogenides. In particular, in the out-of-plane direction, the texturization triggers the lowest lattice thermal conductivity (∼0.39 W K^-1 m^-1), exceeding the theoretical limit of the Debye-Cahill model, which provides a precious opportunity to investigate this real Sb-substituted (SnS)_1.2(TiS₂)₂ material. The present finding in misfit-layered chalcogenides provides a novel strategy for manipulating thermoelectrics via texturization engineering.

High-throughput screening and classification of layered di-metal chalcogenides

Through employing the layered-crystal determination program based on the topology-scaling algorithm, 450 M_mN_nX_x (M, N = metal elements, X = S, Se) layered di-metal chalcogenides (LDCs) are identified from the 1 602 011 crystalline materials known in two Material Genome databases (Materials Project and OQMD). Their structures are classified into three types, 104 compounds in standard M_mN_nX_x homo-layered structures, 34 in M_mX_x1/N_nX_x2 hetero-layered structures and 312 in large-cation M-intercalated N_nX_x layered structures. 312 cation-intercalated LDCs are mostly composed of large cations such as K, Rb, Cs, Ba and Tl, and are not easy to be exfoliated into few-layer 2-dimensional (2D) structures because of the strong ionic bonding between the large cations and the negatively charged layers. In contrast, the homo-layered and hetero-layered structures may be exfoliated into stable few-layer 2D structures due to the weak inter-layer van der Waals interaction. The band structure screening identifies 34 direct- and 108 indirect-band-gap layered semiconductors from the 450 LDCs, and 24 of them have small in-plane effective masses and thus high mobility of hole or electron carriers. Two stable, direct-band-gap and high-mobility mono-layer semiconductors MgAl₂S₄ and ZnIn₂S₄ are found from group II-III₂-VI₄ LDCs. Furthermore, 83 LDCs composed of the magnetic metal elements are found, which provides new platforms for the search of 2D magnetic crystals similar to Cr₂Ge₂Te₆ with intrinsic ferromagnetism. This work extends the search of layered materials from metal dichalcogenides to ternary chalcogenides and can serve as a map for the future discovery of novel 2D semiconductors and magnetic materials.

Fermi- to non-Fermi-liquid crossover and Kondo behavior in two-dimensional (Cu2/3V1/3)V2S4

By means of a specific heat (C) and electrical resistivity ([Formula: see text]) study, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable mass renormalization, [Formula: see text], up to unusually high temperatures  ∼70 K in the layered system (Cu_2/3V_1/3)V₂S₄. At low temperature, a marked upturn of both C and [Formula: see text] is suppressed by magnetic field, which suggests a picture of Kondo coupling between conduction electrons in the VS₂ layers and impurity spins of the V³⁺ ions located between layers. This picture opens the possibility of controlling electronic correlations and the FL to non-FL crossover in simple layered materials. For instance, we envisage that the coupling between layers provided by the impurity spins may realize a two-channel Kondo state.

Solvent exfoliation stabilizes TiS2 nanosheets against oxidation, facilitating lithium storage applications

Titanium disulfide is a promising material for a range of applications, including lithium-ion battery (LIB) anodes. However, its application potential has been severely hindered by the tendency of exfoliated TiS2 to rapidly oxidize under ambient conditions. Herein, we confirm that, although layered TiS2 powder can be exfoliated by sonication in aqueous surfactant solutions, the resultant nanosheets oxidise almost completely within hours. However, we find that upon performing the exfoliation in the solvent cyclohexyl-pyrrolidone (CHP), the oxidation is almost completely suppressed. TiS2 nanosheets dispersed in CHP and stored at 4 °C in an open atmosphere for 90 days remained up to 95% intact. In addition, CHP-exfoliated nanosheets did not show any evidence of oxidation for at least 30 days after being transformed into dry films even when stored under ambient conditions. This stability, probably a result of a residual CHP coating, allows TiS2 nanosheets to be deployed in applications. To demonstrate this, we prepared lithium ion battery anodes from nano : nano composites of TiS2 nanosheets mixed with carbon nanotubes. These anodes displayed reversible capacities (920 mA h g-1) close to the theoretical value and showed good rate performance and cycling capability.

Thickness-Dependent Characterization of Chemically Exfoliated TiS2 Nanosheets

Monolayer TiS₂ is the lightest member of the transition metal dichalcogenide family with promising applications in energy storage and conversion systems. The use of TiS₂ has been limited by the lack of rapid characterization of layer numbers via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate the layer-number-dependent Raman modes for TiS₂. 1T TiS₂ presents two characteristics of the Raman active modes, A_1g (out-of-plane) and E_g (in-plane). We identified a characteristic peak frequency shift of the E_g mode with the layer number and an unexplored Raman mode at 372 cm^-1 whose intensity changes relative to the A_1g mode with the thickness of the TiS₂ sheets. These two characteristic features of Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS₂. Further, we develop a method to produce oxidation-resistant inks of micron-sized mono- and few-layered TiS₂ nanosheets at concentrations up to 1 mg/mL. These TiS₂ inks can be deposited to form thin films with controllable thickness and nanosheet density over square centimeter areas. This opens up pathways for a wider utilization of exfoliated TiS₂ toward a range of applications.

Ultrafast saturable absorption in TiS2 induced by non-equilibrium electrons and the generation of a femtosecond mode-locked laser

Non-equilibrium electrons induced by ultrafast laser excitation in a correlated electron material can disturb the Fermi energy as well as optical nonlinearity. Here, non-equilibrium electrons translate a semiconductor TiS2 material into a plasma to generate broad band nonlinear optical saturable absorption with a sub-picosecond recovery time of ∼768 fs (corresponding to modulation frequencies over 1.3 THz) and a modulation response up to ∼145%. Based on this optical nonlinear modulator, a stable femtosecond mode-locked pulse with a pulse duration of ∼402 fs and a pulse train with a period of ∼175.5 ns is observed in the all-optical system. The findings indicate that non-equilibrium electrons can promote a TiS2-based saturable absorber to be an ultrafast switch for a femtosecond pulse output.

A study of electron and thermal transport in layered titanium disulphide single crystals

We present a detailed study of thermal and electrical transport behavior of single crystal titanium disulphide flakes, which belong to the two dimensional, transition metal dichalcogenide class of materials. In-plane Seebeck effect measurements revealed a typical metal-like linear temperature dependence in the range of 85-285 K. Electrical transport measurements with in-plane current geometry exhibited a nearly T ² dependence of resistivity in the range of 42-300 K. However, transport measurements along the out-of-plane current geometry showed a transition in temperature dependence of resistivity from T ² to T ⁵ beyond 200 K. Interestingly, Au ion-irradiated TiS₂ samples showed a similar T ⁵ dependence of resistivity beyond 200 K, even in the current-in-plane geometry. Micro-Raman measurements were performed to study the phonon modes in both pristine and ion-irradiated TiS₂ crystals.

The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2

A family of phases, Co _x TiS₂ (0 ≤ x ≤ 0.75) has been prepared and characterised by powder X-ray and neutron diffraction, electrical and thermal transport property measurements, thermal analysis and SQUID magnetometry. With increasing cobalt content, the structure evolves from a disordered arrangement of cobalt ions in octahedral sites located in the van der Waals' gap (x ≤ 0.2), through three different ordered vacancy phases, to a second disordered phase at x ≥ 0.67. Powder neutron diffraction reveals that both octahedral and tetrahedral inter-layer sites are occupied in Co_0.67TiS₂. Charge transfer from the cobalt guest to the TiS₂ host affords a systematic tuning of the electrical and thermal transport properties. At low levels of cobalt intercalation (x < 0.1), the charge transfer increases the electrical conductivity sufficiently to offset the concomitant reduction in |S|. This, together with a reduction in the overall thermal conductivity leads to thermoelectric figures of merit that are 25% higher than that of TiS₂, ZT reaching 0.30 at 573 K for Co _x TiS₂ with 0.04 ≤ x ≤ 0.08. Whilst the electrical conductivity is further increased at higher cobalt contents, the reduction in |S| is more marked due to the higher charge carrier concentration. Furthermore both the charge carrier and lattice contributions to the thermal conductivity are increased in the electrically conductive ordered-vacancy phases, with the result that the thermoelectric performance is significantly degraded. These results illustrate the competition between the effects of charge transfer from guest to host and the disorder generated when cobalt cations are incorporated in the inter-layer space.

Searching for new thermoelectric materials: some examples among oxides, sulfides and selenides

Different families of thermoelectric materials have been investigated since the discovery of thermoelectric effects in the mid-19th century, materials mostly belonging to the family of degenerate semi-conductors. In the last 20 years, new thermoelectric materials have been investigated following different theoretical proposals, showing that nanostructuration, electronic correlations and complex crystallographic structures (low dimensional structures, large number of atoms per lattice, presence of 'rattlers'…) could enhance the thermoelectric properties by enhancing the Seebeck coefficient and/or reducing the thermal conductivity. In this review, the different strategies used to optimize the thermoelectric properties of oxides and chalcogenides will be presented, starting with a review on thermoelectric oxides. The thermoelectric properties of sulfides and selenides will then be discussed, focusing on layered materials and low dimensional structures (TiS2 and pseudo-hollandites). Some sulfides with promising ZT values will also be presented (tetrahedrites and chalcopyrites).

On the effects of substitution, intercalation, non-stoichiometry and block layer concept in TiS2 based thermoelectrics

TiS₂ based layered sulfides have recently received increasing interest from the thermoelectric community.

First-principles study on thermodynamic properties and phase transitions in TiS(2)

Structural and vibrational properties of TiS(2) with the CdI(2) structure have been studied to high pressures from density functional calculations with the local density approximation (LDA). The calculated axial compressibility of the CdI(2)-type phase agrees well with experimental data and is typical of layered transition-metal dichalcogenides. The obtained phonon dispersions show a good correspondence with available experiments. A phonon anomaly is revealed at 0 GPa, but is much reduced at 20 GPa. The thermodynamic properties of this phase were also calculated at high pressures and high temperatures using the quasi-harmonic approximation. Our LDA study on the pressure-induced phase transition sequence predicts that the CdI(2)-type TiS(2), the phase stable at ambient conditions, should transform to the cotunnite phase at 15.1 GPa, then to a tetragonal phase (I4/mmm) at 45.0 GPa. The tetragonal phase remains stable to at least 500 GPa. The existence of the tetragonal phase at high pressures is consistent with our previous findings in NiS(2) (Yu and Ross 2010 J. Phys.: Condens. Matter 22 235401). The cotunnite phase, although only stable in a narrow pressure range between 15.1 and 45.0 GPa, displays the formation of a compact S network between 100 and 200 GPa, which is evidenced by a kink in the variation of unit cell lengths with pressure. The electron density analysis in cotunnite shows that valence electrons are delocalized from Ti atoms and concentrated near the S network.