Scanning Tunneling Spectroscopy Study Research Articles

Bandgap study of quantum dot-sized SiGe alloy nanocrystals prepared in a nonthermal capacitively-coupled plasma by ambient scanning tunneling spectroscopy

The manuscript aims to elucidate the evolution of the bandgap and the semiconducting nature of quantum dot-sized SiGe alloy nanocrystals prepared at different plasma conditions in VHF PECVD. The technique of scanning tunneling spectroscopy (STS) is used for this, considering its highly localized nature, and to study the individual NCs. The STS analysis gives the density of states (DOS) spectrum of the SiGe alloy NCs. The present STS study reveals a dependence of the bandgap of SiGe alloy NCs on plasma parameters. The SiGe alloy NCs also exhibited a shallow p-type character despite the fact that no dopants were used during the deposition. Besides this, the midgap defect states are also observed in some of the samples. Further, a comparative bandgap study of the SiGe alloy NCs with its bulk indicates quantum confinement. These new findings from the QD-sized SiGe alloy NCs are important for the conceptual design and practical development of quantum dot-based third-generation solar cells.

Twist-Angle-Dependent Electronic Properties of Exfoliated Single Layer MoS2 on Au(111).

Synthetic materials and heterostructures obtained by the controlled stacking of exfoliated monolayers are emerging as attractive functional materials owing to their highly tunable properties. We present a detailed scanning tunneling microscopy and spectroscopy study of single layer MoS2-on-gold heterostructures as a function of the twist angle. We find that their electronic properties are determined by the hybridization of the constituent layers and are modulated at the moiré period. The hybridization depends on the layer alignment, and the modulation amplitude vanishes with increasing twist angle. We explain our observations in terms of a hybridization between the nearest sulfur and gold atoms, which becomes spatially more homogeneous and weaker as the moiré periodicity decreases with increasing twist angle, unveiling the possibility of tunable hybridization of electronic states via twist angle engineering.

Gate-Tunable Renormalization of Spin-Correlated Flat-Band States and Bandgap in a 2D Magnetic Insulator.

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between exchange and Coulomb interactions. The ability to precisely tune the Coulomb interaction enables the control of spin-correlated flat-band states, band gap, and unconventional magnetism in such strongly correlated materials. Here, we demonstrate a gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr3) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the interflat-band spacing and bandgap of CrBr3 can be continuously tuned by 120 and 240 meV, respectively, via electrostatic injection of carriers into the hybrid CrBr3/graphene system. This can be attributed to the self-screening of CrBr3 arising from the gate-induced carriers injected into CrBr3, which dominates over the weakened remote screening of the graphene substrate due to the decreased carrier density in graphene. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides effective strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.

Two-Dimensional Magnetic Semiconducting Heterostructures of Single-Layer CrI3-CrI2.

Single-layer heterostructures of magnetic materials are unique platforms for studying spin-related phenomena in two dimensions (2D) and have promising applications in spintronics and magnonics. Here, we report the fabrication of 2D magnetic lateral heterostructures consisting of single-layer chromium triiodide (CrI3) and chromium diiodide (CrI2). By carefully adjusting the abundance of iodine based on molecular beam epitaxy, single-layer CrI3-CrI2 heterostructures were grown on Au(111) surfaces with nearly atomic-level seamless boundaries. Two distinct types of interfaces, i.e., zigzag and armchair interfaces, have been identified by means of scanning tunneling microscopy. Our scanning tunneling spectroscopy study combined with density functional theory calculations indicates the existence of spin-polarized ground states below and above the Fermi energy localized at the boundary. Both the armchair and zigzag interfaces exhibit semiconducting nanowire behaviors with different spatial distributions of density of states. Our work presents a novel low-dimensional magnetic system for studying spin-related physics with reduced dimensions and designing advanced spintronic devices.

Coexistence of reconstructed and unreconstructed structures in the structural transition regime of twisted bilayer graphene

In twisted bilayer graphene (TBG), a twist-angle-dependent competition between interlayer stacking energy and intralayer elastic energy results in flat rigid layers at large twist angles and lattice reconstruction at small twist angles. Despite enormous scientific interest and effort in the TBG, however, an experimental study of evolution from the rigid lattice to the reconstructed lattice as a function of twist angle is still missing. Here we present a scanning tunneling microscopy and spectroscopy study to reveal the twist-angle-dependent lattice reconstruction in the TBG. Our experiment demonstrates that there is a transition regime between the rigid regime and the relaxed regime, and the reconstructed and unreconstructed structures can coexist in the transition regime. The coexistence of the two distinct structures in this regime may arise from subtle balance between the interlayer stacking energy and intralayer elastic energy in the TBG with intermediate moir\'e sizes. Our results provide an explanation for inconsistent phase diagrams and tunneling spectra reported in TBG around the magic angle.

Alloyed transition-metal dichalcogenides (Mo1−xWxSe2) through a hydrothermal synthesis route: Probing layer-number-dependent band energies and band-gap bowing via scanning tunneling spectroscopy

In this work, we form alloyed transition-metal dichalcogenides (TMDs), ${\mathrm{Mo}}_{1--x}{\mathrm{W}}_{x}{\mathrm{Se}}_{2}$, through a hydrothermal synthesis procedure. Due to the identical effective ionic radius of the cations, the common-anionic alloys did not experience any lattice strain. A complete series of the common-anionic TMD alloys could thereby be synthesized; their nanosheets were formed by a liquid exfoliation method. The electronic properties of the alloyed nanosheets in their monolayer, bilayer, and trilayer forms were recorded separately through scanning tunneling spectroscopy (STS) in an extremely localized manner. From the STS studies, which have a correspondence to the density of states of the semiconductors, we have deliberated on their electronic properties and commented on the band edges of the nanosheets in the alloy series. The transport gap of the alloys in their monolayer, bilayer, and trilayer forms exhibited band-gap bowing, and also a layer-number-dependent bowing coefficient. That is, the bowing phenomenon interestingly occurred only in the few-layered TMD alloys and diminished to finally vanish in thicker nanosheets. The conduction band has been found to be responsible for such a nonlinear behavior of the transport gap; the results could be explained in terms of the molecular orbitals which form the band. The results have established a coherent description of the band-gap tuning in atomically thin 2D TMD alloys.

Influence of self-doping on band-edges and Fermi energy of CsPbBr3

The electronic properties of CsPbBr 3 thin films cast through a solution route are known to depend on the stoichiometry of the precursors. In this work, with the aid of Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS) studies, a complete energy landscape has been derived for the all-inorganic perovskite formed with a wide range of precursor stoichiometries. While the optical band gap of the perovskite is known to remain unchanged with a variation in the stoichiometry, we show the manner in which their Fermi energy, valence band (VB), and conduction band (CB) edges shift. The variation in the energy levels has been correlated to the presence of defects. Since the formation energy of different intrinsic point defects depends on the growth environment, the nature of defects has depended on the precursor stoichiometry used during the formation of the perovskites. The presence of defects has thereby tuned the Fermi energy and the type of electronic conductivity in CsPbBr 3 as well. Here, the CB and VB edges have responded due to the self-doping process. Finally, we have presented the energy landscape of the perovskite with a variation in the precursor stoichiometry, since the energy levels are indeed crucial in designing any optoelectronic devices based on CsPbBr 3 . • Fermi level and band edges of CsPbBr 3 are affected by the precursor stoichiometry. • Energy landscape of the perovskite derived from a combination of KPFM and STS. • CsPbBr 3 formed with CsBr-rich precursors has p -type conductivity. • CsPbBr 3 formed with PbBr 2 -rich growth condition exhibits an n -type behavior. • Bands moved away from vacuum level in perovskites formed in PbBr 2 -rich environment.

Formation of CuIn(1-x)GaxS2 Thin Films through a Solution Approach: Nonlinear Variation of Fermi Energy and Band Gap Bowing.

We report the formation of CuIn(1-x)GaxS2 (CIGS) thin films through a solution approach, namely, successive ionic layer adsorption and reaction (SILAR) technique. The obtained films possessed a high degree of crystallinity indicating the efficacy of the deposition process in forming CIGS films. A series of alloys have evidenced band gap bowing, that is, the optical band gap does not follow a linear relationship with the composition; the band gap of an intermediate compound is higher than that is interpolated from a linear relationship or Vegard's law. The composition-dependent band gap followed a quadratic relationship evidencing reverse band gap bowing, manifesting an upward convex behavior. With scanning tunneling spectroscopy (STS) and thereby the density of states of the disordered semiconductors, we have observed a bowing behavior in the transport gap as well and identified the roles of the conduction and valence bands in yielding the bowing phenomenon. The results are explained in terms of the anionic and cationic orbitals involved in forming the two bands. The STS studies have been analyzed further to derive the Urbach energy of the disordered semiconductors. When STS studies are combined with Kelvin probe force microscopy, which in effect provides the Fermi energy of the alloyed semiconductors, we could derive the band edges and the Fermi energy of the whole series in an absolute energy scale during the manifestation of the bowing phenomenon.

Boron nanostructure formation on Mo(112) surface

Boron-based nanoscale structures are at the vanguard of current research due to multifarious applications in numerous fields. Here we report the formation of ordered boron structure on Mo(112), which was investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While Boron did not form ordered structures at room temperature, it showed a c(2 × 2) structure upon heating at 1500 K. The c(2 × 2) structure shows a regular array of boron clusters on the trench of Mo(112), and STM study does not indicate any alloying. Dynamical LEED analysis and density functional theory reveal that the c(2 × 2) structure is an array of boron clusters, B4. The formation of B4 cluster is also explained by favorable adsorption sites on the trench of Mo(112). The scanning tunneling spectroscopy study shows the metallic nature of this B nanostructure, which will be stimulating for practical applications. This novel network formation by boron clusters indicates strong inter-atomic as well as intra-cluster bonding.

Imaging gate-tunable Tomonaga-Luttinger liquids in 1H-MoSe2 mirror twin boundaries.

One-dimensional electron systems exhibit fundamentally different properties than higher-dimensional systems. For example, electron-electron interactions in one-dimensional electron systems have been predicted to induce Tomonaga-Luttinger liquid behaviour. Naturally occurring grain boundaries in single-layer transition metal dichalcogenides exhibit one-dimensional conducting channels that have been proposed to host Tomonaga-Luttinger liquids, but charge density wave physics has also been suggested to explain their behaviour. Clear identification of the electronic ground state of this system has been hampered by an inability to electrostatically gate such boundaries and tune their charge carrier concentration. Here we present a scanning tunnelling microscopy and spectroscopy study of gate-tunable mirror twin boundaries in single-layer 1H-MoSe2 devices. Gating enables scanning tunnelling microscopy and spectroscopy for different mirror twin boundary electron densities, thus allowing precise characterization of electron-electron interaction effects. Visualization of the resulting mirror twin boundary electronic structure allows unambiguous identification of collective density wave excitations having two velocities, in quantitative agreement with the spin-charge separation predicted by finite-length Tomonaga-Luttinger liquid theory.

Coexistence of Quasi-two-dimensional Superconductivity and Tunable Kondo Lattice in a van der Waals Superconductor

Realization of Kondo lattice in superconducting van der Waals materials not only provides a unique opportunity for tuning the Kondo lattice behavior by electrical gating or intercalation, but also is helpful for further understanding the heavy fermion superconductivity. Here we report a low-temperature and vector-magnetic-field scanning tunneling microscopy and spectroscopy study on a superconducting compound (4Hb-TaS2) with alternate stacking of 1T-TaS2 and 1H-TaS2 layers. We observe the quasi-two-dimensional superconductivity in the 1H-TaS2 layer with anisotropic response to the in-plane and out-of-plane magnetic fields. In the 1T-TaS2 layer, we detect the Kondo resonance peak that results from the Kondo screening of the unpaired electrons in the Star-of-David clusters. We also find that the intensity of the Kondo resonance peak is sensitive to its relative position with the Fermi level, and it can be significantly enhanced when it is further shifted towards the Fermi level by evaporating Pb atoms onto the 1T-TaS2 surface. Our results not only are important for fully understanding the electronic properties of 4Hb-TaS2, but also pave the way for creating tunable Kondo lattice in the superconducting van der Waals materials.

Scanning tunneling microscopy of Bi2Te3 films prepared by pulsed laser deposition: from nanocrystalline structures to van der Waals epitaxy

Bi2Te3 is a material with high efficiency of thermoelectric energy conversion. Recently, it was also recognized as a topological insulator, and is often used as the basis for creation of other types of topological matter. Pulsed laser deposition (PLD) is widely considered as a simple method for growing of multicomponent films, but not as a tool for van der Waals epitaxy. We demonstrate here that the van der Waals epitaxy of Bi2Te3 is indeed impossible in vacuum PLD, but is possible in the presence of a background gas, which is confirmed by the results of scanning tunneling microscopy and spectroscopy studies. Results of ab initio calculations reproduce tunneling spectra of the first three terraces of epitaxial films of Bi2Te3. In addition, an unusual hexagonal superstructure resembling a charge-density wave is observed in overheated films.

In-situ STS studies and first principles calculations on bare and Sn adsorbed UHV exfoliated WS2 layers

Abstarct. Two dimensional (2D) derivatives of tin (Sn) have obtained special deliberations recently due to practical realization of planar, as well as, buckled hexagonal lattice of Sn called stanene. However, it has been observed that proper choice of substrate is very important for growth of stanene like films owing to large core size of Sn that prefers sp 3 hybridization over sp 2. Transition metal dichalcogenides (TMDs) like MoS2 or WS2 with honey comb lattice structure seem to be promising substrate candidates for 2D growth of Sn. In the present work, we report mechanical exfoliation of few layers of WS2 under ultra-high vacuum (UHV) conditions and investigations of growth and local electronic structure by in-situ scanning tunneling microscopy (STM) and spectroscopy (STS) studies. Flat WS2 surface with honeycomb lattice structure in the atomic scale with a lattice constant of 0.34 nm is evident in the STM investigations, whereas, STS measurements reveal local density of states (LDOS) of WS2 with a bandgap of approximately 1.34 eV. Density functional theory (DFT) calculations performed by considering bulk WS2 reveal conduction and valence band states comprised of S p and W d at both sides of the Fermi energy (EF) and an indirect bandgap of 1.38 eV. Experimental observations upon Sn adsorption, reveal commensurate growth of Sn atoms on the sulfur `S’ sites with a buckling height of 40 ±10 pm. STS measurements exhibit local electronic structure of the Sn adsorbed surface with clear evidence of in-gap states. DFT calculations quantify the experimental results demonstrating `S’ sites as the most stable sites for the atomic adsorption of Sn with a buckling height of around 80 pm and reveal signature of in-gap hybridized states comprised of Sn p and W d orbitals.

Necessity of Quantifying Urbach Energy through Scanning Tunneling Spectroscopy.

In this Letter, we introduce scanning tunneling spectroscopy (STS) to quantify the Urbach energy (EU) in disordered semiconductors. The technique enabled us to gain precise information on the extending component of conduction and valence band-edges responsible for Urbach tailing, individually; such information has been obtained from the width of band-energy-histograms drawn from STS studies at many different points. STS, as a probing method at the microscopic scale to derive EU, is in contrast to commonly employed optical spectroscopy studies which provide information at the macroscopic scale. A comparison between Urbach energy values from optical studies and distribution of band-edges obtained from STS revealed the inherent inaccuracies involved in the optical characterization process. We have considered copper oxide (CuxO) thin films in this regard; we show that through STS and the associated density of state (DOS) spectra, we can derive accurate information on the band-edges' distribution leading to EU in different phases of the binary oxide thin films.

Decoration of graphene sheets with silver nanoparticles and their characterization

Graphene has been an emerging area of research since last decade. This fantastic material promises a wide variety of applications starting from electronics to biomedical fields. The properties of graphene can be enhanced by conjugating it with various other elements. The present work describes about the deposition of silver nano-particles via chemical synthesis onto in-house synthesized graphene nanosheets. It can be pointed out that application of infrared accelerates the synthesis process. Few layered graphene nanosheets (FLGNs) has been synthesized through electrochemical exfoliation technique. The colloidal graphene solution stabilized by citrate ions helps in the chemical reduction of AgNO3, which is essential in the formation of a nano-sized silver particulate layer (Ag NPs) over the graphene sheets. The presence of Ag NPs over the as-synthesized graphene was established by XRD, UV–VIS, FE-SEM and STM studies. The change in colloidal stability for the as-synthesized graphene and Ag NPs coated graphene was explained by the dynamic light scattering studies. The electronic characteristics of graphene coated Ag NPs was studied by scanning tunneling spectroscopy (STS) technique. The STS studies revealed that graphene with various numbers of coupled layers is responsible for the variations in the density of states (DOS) pattern. For Ag NPs-FLGNs, the spatial variations in the DOS spectra can be correlated to the size effects of the Ag NPs over the FLGNs. The local density of states (LDOS) spectra originates from the quantized electronic structure of the Ag NPs–FLGs system.

Visualizing Spatial Evolution of Electron-Correlated Interface in Two-Dimensional Heterostructures.

Microscopically visualizing the evolution of electronic structures at the interface between two electron-correlated domains shows fundamental importance in both material science and physics. Here, we report scanning tunneling microscopy and spectroscopy studies of the interfacial electronic structures evolution in a phase-engineered monolayer NbSe2 heterostructure. The H-NbSe2 metallic state penetrates the Mott insulating T-NbSe2 at the H/T phase interface, with a prominent 2D charge density wave (CDW) proximity effect. Moreover, an insulating Mott gap collapse with the disappearance of the upper Hubbard band is detected at the electronic phase transition region. Theoretical calculations reveal that such insulating Mott gap collapse can be attributed to the electron doping effect induced by the interface. Our findings promote a microscopical understanding of the interactions between different electron-correlated systems and provide an effective method for controlling the Mott insulating states with phase engineering.

Energy landscape in silver-bismuth-iodide rudorffites: Combining scanning tunneling spectroscopy and Kelvin probe force microscopy

We report the energy landscape of an upcoming solar cell material series, namely, silver-bismuth-iodide rudorffites $({A}_{m}{B}_{n}{{X}_{m}}_{+3n})$. We formed the compounds through compositional engineering and characterized them. We employed Kelvin probe force microscopy (KPFM) to derive the surface work function and thereby the Fermi energy; scanning tunneling spectroscopy (STS), on the other hand, provided conduction and valence band edges with respect to the Fermi energy of the rudorffites. By combining KPFM and STS studies, we obtained a composition-dependent tuning of the Fermi energy in rudorffite thin films, namely, ${\mathrm{Ag}}_{3}\mathrm{Bi}{\mathrm{I}}_{6}$, ${\mathrm{Ag}}_{2}\mathrm{Bi}{\mathrm{I}}_{5}$, $\mathrm{AgBi}{\mathrm{I}}_{4}$, $\mathrm{Ag}{\mathrm{Bi}}_{2}{\mathrm{I}}_{7}$, and $\mathrm{Ag}{\mathrm{Bi}}_{3}{\mathrm{I}}_{10}$ with a gradual change in electronic conductivity from $p$ to $n$ type. While such behavior has been correlated to point defects, which are possible to form in the different members of the rudorffite series, we more importantly show that a combination of KPFM and STS studies can provide the complete energy landscape of a semiconductor.

Origin of bandgap bowing in Cs2Na1−x Ag x BiCl6 double perovskite solid-state alloys: a paradigm through scanning tunneling spectroscopy

Bandgap bowing has recently been emerged as an effective strategy toward band-engineering in metal halide perovskites. In this work, we report extensive studies on the bowing phenomenon in Cs2NaBiCl6 double perovskite upon alloying with silver at the sodium site. Through optical spectroscopy, composition-dependent bandgap in Cs2Na1−x Ag x BiCl6 (0 ⩽ x ⩽ 1) evidenced bandgap bowing with an upward-concave nature. Further from the quadratic fit, the bowing coefficient (b = 0.74 eV) turned out to be independent of composition and is close to the theoretically predicted value. From scanning tunneling spectroscopy and associated studies on band-energies, we have observed that the bandgap-lowering originated from a significant change of the valence-band position. The behavior of band-energies has been explained through the orbital-contributions of the constituent elements responsible in forming the two bands. Formation of an internal type-I band-alignment between the two end-members, namely Cs2NaBiCl6 and Cs2AgBiCl6 could be visualized. Based on experimental evidences, we could infer that the bow-like evolution of bandgap in Cs2Na1−x Ag x BiCl6 alloys is principally dominated by structural distortions in the Cs2NaBiCl6 host-lattice upon incorporation of silver.

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions.

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene.

Organic charge-transfer complexes (CTCs) formed by strong electron acceptor and strong electron donor molecules are known to exhibit exotic effects such as superconductivity and charge density waves. We present a low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated tetracyanoquinodimethane (F4TCNQ), self-assembled on the surface of oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the formation of the charge-transfer complex by dI/dV spectroscopy and direct imaging of the singly occupied molecular orbitals. High-resolution spectroscopy reveals a gap at zero bias, suggesting the formation of a correlated ground state at low temperatures. These results point to the possibility to realize and study correlated ground states in charge-transfer complex monolayers on weakly interacting surfaces.