Direct observation of strain-induced orbital valence band splitting in 
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 by sodium intercalation

Eknapakul, Tanachat; Fongkaew, Ittipon; Siriroj, S.; Jindata, Warakorn; Chaiyachad, Sujinda; Mo, S.-K.; Thakur, Sangeeta; Petaccia, L.; Takagi, H.; Limpijumnong, Sukit; Meevasana, W.

doi:10.1103/physrevb.97.201104

Cited by 14 publications

(9 citation statements)

References 59 publications

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“…A recurrent question in the study of alkali-metal dosed surfaces of 2D materials is whether the dosed atoms remain on the surface and set up a vertical electric field, or intercalate into the sample [1][2][3][4][6][7][8][9][10][11][12][13][38][39][40][41][42]. A number of recent studies on TMDCs and similar materials have claimed that the alkali metal atoms remain on the surface [2][3][4][5][6]8,9,42], in the case that the sample remains at low temperatures throughout.…”

Section: Resultsmentioning

confidence: 99%

“…The alternative scenario is intercalation, in which the K atoms migrate into the van der Waals gap between layers, leading to an increased van der Waals gap that causes decoupling of the topmost layer both structurally and electronically. This scenario is known to occur almost immediately at room temperature [1] but is also often applied to dosing studies of TMDCs even when the sample is held at low temperatures [7,10,40]. This topmost layer also becomes doped due to the donated electrons from the K + ions.…”

Section: Resultsmentioning

confidence: 99%

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Bulk and surface electronic states in the dosed semimetallic HfTe2

et al. 2020

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The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T-HfTe 2 , a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of potassium (K) deposition. From the k z maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wave-function confinement at the topmost layer. In our highest-dosing cases, a monolayerlike electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below E F , we can directly measure a band overlap of ∼0.2 eV. However, 3D bulklike states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Bulk and surface electronic states in the dosed semimetallic HfTe2

et al. 2020

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“… 35 Since the charge density wave transition temperature of HfSe 2 is rather low, we can exclude this interpretation here. It has been shown that HfSe 2 is a suitable host material for extrinsic dopant atoms, 36 suggesting that the bright defects might be a result of the intercalation of impurities.…”

Section: Resultsmentioning

confidence: 99%

Nanoscale Investigation of Defects and Oxidation of HfSe₂

et al. 2018

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HfSe2 is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe2. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe2 surface using scanning tunneling microscopy and spectroscopy. Compared to MoS2 and WSe2, HfSe2 exhibits similar type of defects, albeit with a substantially higher density of 9 × 1011 cm–2. The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe2 surface leads to very low Schottky barrier heights. Conductive atomic force microscopy measurements reveal a very small dependence of the Schottky barrier height on the work function of the metals, suggesting a strong Fermi-level pinning. We attribute the observed Fermi-level pinning (pinning factor ∼0.1) to surface distortions and Se/Hf defects. In addition, we have also studied the HfSe2 surface after the exposure to air by scanning tunneling microscopy and conductive atomic force microscopy. Partly oxidized layers with band gaps of 2 eV and Schottky barrier heights of ∼0.6 eV were readily found on the surface. Our experiments reveal that HfSe2 is very air-sensitive, implying that capping or encapsulating of HfSe2, in order to protect it against oxidation, is a necessity for technological applications.

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“…But also resistivity [40,[47][48][49], conductivity [3,43] Hall coefficient, [47], magnetic susceptibility [3], scanning transmission electron microscopy [39], and electron energy-loss [50,51] experiments were performed. The investigations were supported by numerous theoretical approaches [11,14,27,31,38,43,[52][53][54][55][56][57][58][59].…”

Section: Introductionmentioning

confidence: 97%

“…Besides transition metals [60][61][62], which have been intercalated into both materials, alkali metals have been used as electron donors for HfSe 2 [63,64]. Lithium [65,66] and sodium [6,56] doping leads to a transition from semiconducting to metallic behavior. In the pristine materials, the valence states are mainly comprised of S/Se p-orbitals which are almost filled due to a transfer of electrons from the hafnium atoms [67].…”

Section: Introductionmentioning

confidence: 99%

Potassium-intercalated bulk HfS$_2$ and HfSe$_2$: Phase stability, structure, and electronic structure

Habenicht,

Simon,

Richter

et al. 2020

Preprint

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We have studied potassium-intercalated bulk HfS2 and HfSe2 by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. The results reveal insights into (1) the intercalation process itself, (2) its effect on the crystal structures, (3) the induced semiconductor-to-metal transitions, and (4) the accompanying appearance of charge carrier plasmons and their dispersions.Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations x are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium.At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high x while the crystal layer thicknesses expand by 33 − 36 %. Calculations suggest that this expansion reaches its maximum at doping levels of x ≈ 0.25 before it reverses slightly for higher concentrations. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1 %.The introduction of potassium causes the formation of charge carrier plasmons whose nature we confirmed by calculating the loss functions and their intraband and interband contributions. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy.The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for x < 0.6.The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.

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Direct observation of strain-induced orbital valence band splitting in HfSe2 by sodium intercalation

Cited by 14 publications

References 59 publications

Bulk and surface electronic states in the dosed semimetallic HfTe2

Bulk and surface electronic states in the dosed semimetallic HfTe2

Nanoscale Investigation of Defects and Oxidation of HfSe₂

Potassium-intercalated bulk HfS$_2$ and HfSe$_2$: Phase stability, structure, and electronic structure

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Direct observation of strain-induced orbital valence band splitting in HfSe2 by sodium intercalation

Cited by 14 publications

References 59 publications

Bulk and surface electronic states in the dosed semimetallic HfTe2

Bulk and surface electronic states in the dosed semimetallic HfTe2

Nanoscale Investigation of Defects and Oxidation of HfSe2

Potassium-intercalated bulk HfS$_2$ and HfSe$_2$: Phase stability, structure, and electronic structure

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Product

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Nanoscale Investigation of Defects and Oxidation of HfSe₂