Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi2 Te4

Quantum anomalous Hall (QAH) insulators transport charge without resistance along topologically protected chiral 1D edge states. Yet, in magnetic topological insulators to date, topological protection is far from robust, with zero-magnetic field QAH effect only realized at temperatures an order of magnitude below the Néel temperature TN , though small magnetic fields can stabilize QAH effect. Understanding why topological protection breaks down is therefore essential to realizing QAH effect at higher temperatures. Here a scanning tunneling microscope is used to directly map the size of exchange gap (Eg,ex ) and its spatial fluctuation in the QAH insulator 5-layer MnBi2 Te4 . Long-range fluctuations of Eg,ex are observed, with values ranging between 0 (gapless) and 70 meV, appearing to be uncorrelated to individual surface point defects. The breakdown of topological protection is directly imaged, showing that the gapless edge state, the hallmark signature of a QAH insulator, hybridizes with extended gapless regions in the bulk. Finally, it is unambiguously demonstrated that the gapless regions originate from magnetic disorder, by demonstrating that a small magnetic field restores Eg,ex in these regions, explaining the recovery of topological protection in magnetic fields. The results indicate that overcoming magnetic disorder is the key to exploiting the unique properties of QAH insulators.

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.

Effects of Floquet Engineering on the Coherent Exciton Dynamics in Monolayer WS2

Coherent optical manipulation of electronic bandstructures via Floquet Engineering is a promising means to control quantum systems on an ultrafast time scale. However, the ultrafast switching on/off of the driving field comes with questions regarding the limits of the Floquet formalism (which is defined for an infinite periodic drive) through the switching process and to what extent the transient changes can be driven adiabatically. Experimentally addressing these questions has been difficult, in large part due to the absence of an established technique to measure coherent dynamics through the duration of the pulse. Here, using multidimensional coherent spectroscopy we explicitly excite, control, and probe a coherent superposition of excitons in the K and K' valleys in monolayer WS₂. With a circularly polarized, red-detuned pump pulse, the degeneracy of the K and K' excitons can be lifted, and the phase of the coherence rotated. We directly measure phase rotations greater than π during the 100 fs driving pulse and show that this can be described by a combination of the AC-Stark shift of excitons in one valley and the Bloch-Siegert shift of excitons in the opposite valley. Despite showing a smooth evolution of the phase that directly follows the intensity envelope of the nonresonant pump pulse, the process is not perfectly adiabatic. By measuring the magnitude of the macroscopic coherence as it evolves before, during, and after the nonresonant pump pulse we show that there is additional decoherence caused by power broadening in the presence of the nonresonant pump. This nonadiabaticity arises as a result of interactions with the otherwise adiabatic Floquet states and may be a problem for many applications, such as manipulating qubits in quantum information processing; however, these measurements also suggest ways such effects can be minimized or eliminated.

Graphene and Beyond: Recent Advances in Two-Dimensional Materials Synthesis, Properties, and Devices

Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field.

Large Magnetic Gap in a Designer Ferromagnet-Topological Insulator-Ferromagnet Heterostructure

Combining magnetism and nontrivial band topology gives rise to quantum anomalous Hall (QAH) insulators and exotic quantum phases such as the QAH effect where current flows without dissipation along quantized edge states. Inducing magnetic order in topological insulators via proximity to a magnetic material offers a promising pathway toward achieving the QAH effect at a high temperature for lossless transport applications. One promising architecture involves a sandwich structure comprising two single-septuple layers (1SL) of MnBi₂ Te₄ (a 2D ferromagnetic insulator) with ultrathin few quintuple layer (QL) Bi₂ Te₃ in the middle, and it is predicted to yield a robust QAH insulator phase with a large bandgap greater than 50 meV. Here, the growth of a 1SL MnBi₂ Te₄ /4QL Bi₂ Te₃ /1SL MnBi₂ Te₄ heterostructure via molecular beam epitaxy is demonstrated and the electronic structure probed using angle-resolved photoelectron spectroscopy. Strong hexagonally warped massive Dirac fermions and a bandgap of 75 ± 15 meV are observed. The magnetic origin of the gap is confirmed by the observation of the exchange-Rashba effect, as well as the vanishing bandgap above the Curie temperature, in agreement with density functional theory calculations. These findings provide insights into magnetic proximity effects in topological insulators and reveal a promising platform for realizing the QAH effect at elevated temperatures.

Defects, band bending and ionization rings in MoS2

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS₂however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy and STS to study embedded sulphur vacancies in bulk MoS₂crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending. The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.

Formation of a Stable Surface Oxide in MnBi2Te4 Thin Films

Understanding the air stability of MnBi₂Te₄ thin films is crucial for the development and long-term operation of electronic devices based on magnetic topological insulators. In the present work, we study MnBi₂Te₄ thin films upon exposure to the atmosphere using a combination of synchrotron-based photoelectron spectroscopy, room-temperature electrical transport, and atomic force microscopy to determine the oxidation process. After 2 days of air exposure, a 2 nm thick oxide passivates the surface, corresponding to the oxidation of only the top two surface layers, with the underlying layers preserved. This protective oxide layer results in samples that still exhibit metallic conduction even after several days of air exposure. Furthermore, the work function decreases from 4.4 eV for pristine MnBi₂Te₄ to 4.0 eV after the formation of the oxide, along with only a small shift in the core levels, indicating minimal doping as a result of air exposure. With the oxide confined to the top surface layers, and the underlying layers preserved, it may be possible to explore new avenues in how to handle, prepare, and passivate future MnBi₂Te₄ devices.

Crossover from 2D Ferromagnetic Insulator to Wide Band Gap Quantum Anomalous Hall Insulator in Ultrathin MnBi2Te4

Intrinsic magnetic topological insulators offer low disorder and large magnetic band gaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi₂Te₄, and measurement of the magnetic energy gap at the Dirac point in ultrathin MnBi₂Te₄ has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from a large band gap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultrathin MnBi₂Te₄. We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a band gap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi₂Te₄. The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

Recent Progress in Proximity Coupling of Magnetism to Topological Insulators

Inducing long-range magnetic order in 3D topological insulators can gap the Dirac-like metallic surface states, leading to exotic new phases such as the quantum anomalous Hall effect or the axion insulator state. These magnetic topological phases can host robust, dissipationless charge and spin currents or unique magnetoelectric behavior, which can be exploited in low-energy electronics and spintronics applications. Although several different strategies have been successfully implemented to realize these states, to date these phenomena have been confined to temperatures below a few Kelvin. This review focuses on one strategy: inducing magnetic order in topological insulators by proximity of magnetic materials, which has the capability for room temperature operation, unlocking the potential of magnetic topological phases for applications. The unique advantages of this strategy, the important physical mechanisms facilitating magnetic proximity effect, and the recent progress to achieve, understand, and harness proximity-coupled magnetic order in topological insulators are discussed. Some emerging new phenomena and applications enabled by proximity coupling of magnetism and topological materials, such as skyrmions and the topological Hall effect, are also highlighted, and the authors conclude with an outlook on remaining challenges and opportunities in the field.

Progress in Epitaxial Thin-Film Na3 Bi as a Topological Electronic Material

Trisodium bismuthide (Na₃ Bi) is the first experimentally verified topological Dirac semimetal, and is a 3D analogue of graphene hosting relativistic Dirac fermions. Its unconventional momentum-energy relationship is interesting from a fundamental perspective, yielding exciting physical properties such as chiral charge carriers, the chiral anomaly, and weak anti-localization. It also shows promise for realizing topological electronic devices such as topological transistors. Herein, an overview of the substantial progress achieved in the last few years on Na₃ Bi is presented, with a focus on technologically relevant large-area thin films synthesized via molecular beam epitaxy. Key theoretical aspects underpinning the unique electronic properties of Na₃ Bi are introduced. Next, the growth process on different substrates is reviewed. Spectroscopic and microscopic features are illustrated, and an analysis of semiclassical and quantum transport phenomena in different doping regimes is provided. The emergent properties arising from confinement in two dimensions, including thickness-dependent and electric-field-driven topological phase transitions, are addressed, with an outlook toward current challenges and expected future progress.

Helical Edge Transport in Millimeter-Scale Thin Films of Na3Bi

A two-dimensional topological insulator (2DTI) has an insulating bulk and helical edges robust to nonmagnetic backscattering. While ballistic transport has been demonstrated in micron-scale 2DTIs, larger samples show significant backscattering and a nearly temperature-independent resistance of unclear origin. Spin polarization has been measured, however the degree of helicity is difficult to quantify. Here, we study 2DTI few-layer Na₃Bi on insulating Al₂O₃. A nonlocal conductance measurement demonstrates edge conductance in the topological regime with an edge mean free path ∼100 nm. A perpendicular magnetic field suppresses spin-flip scattering in the helical edges, resulting in a giant negative magnetoresistance (GNMR) up to 80% at 0.9 T. Comparison to theory indicates >96% of scattering is helical spin scattering significantly exceeding the maximum (67%) expected for a nonhelical metal. GNMR, coupled with nonlocal measurements, thus provides an unambiguous experimental signature of helical edges that we expect to be generically useful in understanding 2DTIs.

Quantum Transport in Air-Stable Na3Bi Thin Films

Na₃Bi has attracted significant interest in both bulk form as a three-dimensional topological Dirac semimetal and ultrathin form as a wide-band gap two-dimensional topological insulator. Its extreme air sensitivity has limited experimental efforts on thin and ultrathin films grown via molecular beam epitaxy to ultrahigh vacuum environments. Here, we demonstrate air-stable Na₃Bi thin films passivated with magnesium difluoride (MgF₂) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF₂ or Si has minimal impact on the transport properties of Na₃Bi while in ultrahigh vacuum. Importantly, the MgF₂-passivated Na₃Bi films are air-stable and remain metallic for over 100 h after exposure to air, as compared to near instantaneous degradation when they are unpassivated. Air stability enables transfer of films to a conventional high-magnetic field cryostat, enabling quantum transport measurements, which verify that the Dirac semimetal character of Na₃Bi films is retained after air exposure.

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes-where organic ligands bind to several metal atoms-are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise-via supramolecular chemistry on a noble metal surface-one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpy's linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal-metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.

Room temperature in-plane ferroelectricity in van der Waals In2Se3

Van der Waals (vdW) assembly of layered materials is a promising paradigm for creating electronic and optoelectronic devices with novel properties. Ferroelectricity in vdW layered materials could enable nonvolatile memory and low-power electronic and optoelectronic switches, but to date, few vdW ferroelectrics have been reported, and few in-plane vdW ferroelectrics are known. We report the discovery of in-plane ferroelectricity in a widely investigated vdW layered material, β'-In₂Se₃. The in-plane ferroelectricity is strongly tied to the formation of one-dimensional superstructures aligning along one of the threefold rotational symmetric directions of the hexagonal lattice in the c plane. Surprisingly, the superstructures and ferroelectricity are stable to 200°C in both bulk and thin exfoliated layers of In₂Se₃. Because of the in-plane nature of ferroelectricity, the domains exhibit a strong linear dichroism, enabling novel polarization-dependent optical properties.

Observation of Effective Pseudospin Scattering in ZrSiS

3D Dirac semimetals are an emerging class of materials that possess topological electronic states with a Dirac dispersion in their bulk. In nodal-line Dirac semimetals, the conductance and valence bands connect along a closed path in momentum space, leading to the prediction of pseudospin vortex rings and pseudospin skyrmions. Here, we use Fourier transform scanning tunneling spectroscopy (FT-STS) at 4.5 K to resolve quasiparticle interference (QPI) patterns at single defect centers on the surface of the line nodal semimetal zirconium silicon sulfide (ZrSiS). Our QPI measurements show pseudospin conservation at energies close to the line node. In addition, we determine the Fermi velocity to be ℏv_F = 2.65 ± 0.10 eV Å in the Γ-M direction ∼300 meV above the Fermi energy E_F and the line node to be ∼140 meV above E_F. More importantly, we find that certain scatterers can introduce energy-dependent nonpreservation of pseudospin, giving rise to effective scattering between states with opposite pseudospin deep inside valence and conduction bands. Further investigations of quasiparticle interference at the atomic level will aid defect engineering at the synthesis level, needed for the development of lower-power electronics via dissipationless electronic transport in the future.

Spatial charge inhomogeneity and defect states in topological Dirac semimetal thin films of Na3Bi

Topological Dirac semimetals (TDSs) are three-dimensional analogs of graphene, with carriers behaving like massless Dirac fermions in three dimensions. In graphene, substrate disorder drives fluctuations in Fermi energy, necessitating construction of heterostructures of graphene and hexagonal boron nitride (h-BN) to minimize the fluctuations. Three-dimensional TDSs obviate the substrate and should show reduced E_F fluctuations due to better metallic screening and higher dielectric constants. We map the potential fluctuations in TDS Na₃Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than the thermal energy room temperature (ΔE_F,rms = 4 to 6 meV = 40 to 70 K) and comparable to the highest-quality graphene on h-BN. Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provide a unique way to tune the surface density of states in a TDS thin-film material. Sparse defect clusters show bound states whose occupation may be changed by applying a bias to the scanning tunneling microscope tip, offering an opportunity to study a quantum dot connected to a TDS reservoir.

Molecular Doping the Topological Dirac Semimetal Na3Bi across the Charge Neutrality Point with F4-TCNQ

We perform low-temperature transport and high-resolution photoelectron spectroscopy on 20 nm thin film topological Dirac semimetal Na3Bi grown by molecular beam epitaxy. We demonstrate efficient electron depletion ∼10(13) cm(-2) of Na3Bi via vacuum deposition of molecular F4-TCNQ without degrading the sample mobility. For samples with low as-grown n-type doping (1 × 10(12) cm(-2)), F4-TCNQ doping can achieve charge neutrality and even a net p-type doping. Photoelectron spectroscopy and density functional theory are utilized to investigate the behavior of F4-TCNQ on the Na3Bi surface.

Strong and Tunable Spin-Orbit Coupling in a Two-Dimensional Hole Gas in Ionic-Liquid Gated Diamond Devices

Hydrogen-terminated diamond possesses due to transfer doping a quasi-two-dimensional (2D) hole accumulation layer at the surface with a strong, Rashba-type spin-orbit coupling that arises from the highly asymmetric confinement potential. By modulating the hole concentration and thus the potential using an electrostatic gate with an ionic-liquid dielectric architecture the spin-orbit splitting can be tuned from 4.6-24.5 meV with a concurrent spin relaxation length of 33-16 nm and hole sheet densities of up to 7.23 × 10(13) cm(-2). This demonstrates a spin-orbit interaction of unprecedented strength and tunability for a 2D hole system at the surface of a wide band gap semiconductor. With a spin relaxation length that is experimentally accessible using existing nanofabrication techniques, this result suggests that hydrogen-terminated diamond has great potential for the study and application of spin transport phenomena.

Electronic Properties of High-Quality Epitaxial Topological Dirac Semimetal Thin Films

Topological Dirac semimetals (TDS) are three-dimensional analogues of graphene, with linear electronic dispersions in three dimensions. Nanoscale confinement of TDSs in thin films is a necessary step toward observing the conventional-to-topological quantum phase transition (QPT) with increasing film thickness, gated devices for electric-field control of topological states, and devices with surface-state-dominated transport phenomena. Thin films can also be interfaced with superconductors (realizing a host for Majorana Fermions) or ferromagnets (realizing Weyl Fermions or T-broken topological states). Here we report structural and electrical characterization of large-area epitaxial thin films of TDS Na3Bi on single crystal Al2O3[0001] substrates. Charge carrier mobilities exceeding 6,000 cm(2)/(V s) and carrier densities below 1 × 10(18) cm(-3) are comparable to the best single crystal values. Perpendicular magnetoresistance at low field shows the perfect weak antilocalization behavior expected for Dirac Fermions in the absence of intervalley scattering. At higher fields up to 0.5 T anomalously large quadratic magnetoresistance is observed, indicating that some aspects of the low field magnetotransport (μB < 1) in this TDS are yet to be explained.

Creating a Stable Oxide at the Surface of Black Phosphorus

The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.

Profound Effect of Substrate Hydroxylation and Hydration on Electronic and Optical Properties of Monolayer MoS2

Atomic force microscopy, Kelvin probe force microscopy, and scanning photoluminescence spectroscopy image the progressive postgrowth hydroxylation and hydration of atomically flat Al2O3(0001) under monolayer MoS2, manifested in large work function shifts (100 mV) due to charge transfer (>10(13) cm(-2)) from the substrate and changes in PL intensity, energy, and peak width. In contrast, trapped water between exfoliated graphene and Al2O3(0001) causes surface potential and doping changes one and two orders of magnitude smaller, respectively, and MoS2 grown on hydrophobic hexagonal boron nitride is unaffected by water exposure.

A graphene field-effect transistor as a molecule-specific probe of DNA nucleobases

Fast and reliable DNA sequencing is a long-standing target in biomedical research. Recent advances in graphene-based electrical sensors have demonstrated their unprecedented sensitivity to adsorbed molecules, which holds great promise for label-free DNA sequencing technology. To date, the proposed sequencing approaches rely on the ability of graphene electric devices to probe molecular-specific interactions with a graphene surface. Here we experimentally demonstrate the use of graphene field-effect transistors (GFETs) as probes of the presence of a layer of individual DNA nucleobases adsorbed on the graphene surface. We show that GFETs are able to measure distinct coverage-dependent conductance signatures upon adsorption of the four different DNA nucleobases; a result that can be attributed to the formation of an interface dipole field. Comparison between experimental GFET results and synchrotron-based material analysis allowed prediction of the ultimate device sensitivity, and assessment of the feasibility of single nucleobase sensing with graphene.

Graphene field effect transistor as a probe of electronic structure and charge transfer at organic molecule-graphene interfaces

The electronic structure of physisorbed molecules containing aromatic nitrogen heterocycles (triazine and melamine) on graphene is studied using a combination of electronic transport, X-ray photoemission spectroscopy and density functional theory calculations. The interfacial electronic structure and charge transfer of weakly coupled molecules on graphene is found to be governed by work function differences, molecular dipole moments and polarization effects. We demonstrate that molecular depolarization plays a significant role in these charge transfer mechanisms even at submonolayer coverage, particularly for molecules which possess strong dipoles. Electronic transport measurements show a reduction of graphene conductivity and charge carrier mobility upon the adsorption of the physisorbed molecules. This effect is attributed to the formation of additional electron scattering sites in graphene by the molecules and local molecular electric fields. Our results show that adsorbed molecules containing polar functional groups on graphene exhibit different coverage behaviour to nonpolar molecules. These effects open up a range of new opportunities for recognition of different molecules on graphene-based sensor devices.

Spin-orbit interaction in a two-dimensional hole gas at the surface of hydrogenated diamond

Hydrogenated diamond possesses a unique surface conductivity as a result of transfer doping by surface acceptors. Yet, despite being extensively studied for the past two decades, little is known about the system at low temperature, particularly whether a two-dimensional hole gas forms at the diamond surface. Here we report that (100) diamond, when functionalized with hydrogen, supports a p-type spin-3/2 two-dimensional surface conductivity with a spin-orbit interaction of 9.74 ± 0.1 meV through the observation of weak antilocalization effects in magneto-conductivity measurements at low temperature. Fits to 2D localization theory yield a spin relaxation length of 30 ± 1 nm and a spin-relaxation time of ∼ 0.67 ± 0.02 ps. The existence of a 2D system with spin orbit coupling at the surface of a wide band gap insulating material has great potential for future applications in ferromagnet-semiconductor and superconductor-semiconductor devices.

Determining the electronic confinement of a subsurface metallic state

Dopant profiles in semiconductors are important for understanding nanoscale electronics. Highly conductive and extremely confined phosphorus doping profiles in silicon, known as Si:P δ-layers, are of particular interest for quantum computer applications, yet a quantitative measure of their electronic profile has been lacking. Using resonantly enhanced photoemission spectroscopy, we reveal the real-space breadth of the Si:P δ-layer occupied states and gain a rare view into the nature of the confined orbitals. We find that the occupied valley-split states of the δ-layer, the so-called 1Γ and 2Γ, are exceptionally confined with an electronic profile of a mere 0.40 to 0.52 nm at full width at half-maximum, a result that is in excellent agreement with density functional theory calculations. Furthermore, the bulk-like Si 3pz orbital from which the occupied states are derived is sufficiently confined to lose most of its pz-like character, explaining the strikingly large valley splitting observed for the 1Γ and 2Γ states.

Direct observation of phonon emission from hot electrons: spectral features in diamond secondary electron emission

In this work we use high-resolution synchrotron-based photoelectron spectroscopy to investigate the low kinetic energy electron emission from two negative electron affinity surfaces of diamond, namely hydrogenated and lithiated diamond. For hydrogen-terminated diamond electron emission below the conduction band minimum (CBM) is clearly observed as a result of phonon emission subsequent to carrier thermalization at the CBM. In the case of lithiated diamond, we find the normal conduction band minimum emission peak is asymmetrically broadened to higher kinetic energies and argue the broadening is a result of ballistic emission from carriers thermalized to the CBM in the bulk well before the onset of band-bending. In both cases the spectra display intensity modulations that are the signature of optical phonon emission as the main mechanism for carrier relaxation. To our knowledge, these measurements represent the first direct observation of hot carrier energy loss via photoemission.

Air-stable electron depletion of Bi(2)Se(3) using molybdenum trioxide into the topological regime

We perform high-resolution photoelectron spectroscopy on in situ cleaved topological insulator Bi2Se3 single crystals and in situ transport measurements on Bi2Se3 films grown by molecular beam epitaxy. We demonstrate efficient electron depletion of Bi2Se3 via vacuum deposition of molecular MoO3, lowering the surface Fermi energy to within ∼100 meV of the Dirac point, well into the topological regime. A 100 nm MoO3 film provides an air-stable doping and passivation layer.

Charge transfer doping of silicon

We demonstrate a novel doping mechanism of silicon, namely n-type transfer doping by adsorbed organic cobaltocene (CoCp2*) molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core-level photoelectron spectroscopy. The concomitant loss of electrons in the CoCp2* adlayer is quantified by the relative intensities of chemically shifted Co2p components in core-level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule-specific parameter, the negative donor energy of -(0.50±0.15)  eV suffices to describe the self-limiting doping process with a maximum areal density of transferred electrons of 2×1013  cm-2 in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addressed.