This work demonstrates a method for 2D thermal mapping of the surface temperature rise of semiconductor devices using thermoreflectance imaging of mechanically exfoliated MoS2 flakes. Thickness-dependent thermoreflectance spectra were acquired, and the A, B, and C excitons of MoS2 were shown to have high thermoreflectance responses. The A exciton, corresponding to a probing wavelength of 675 nm, had the highest thermoreflectance response, but temperature rise (Delta T) measurements using this wavelength can be prone to nonlinear effects; nevertheless, these effects can be accounted for through calibration. Additionally, the high thermoreflectance response enables high measurement sensitivity for devices with low Delta T (e.g., <10 K), such as diamond-based electronics. The C exciton (probing wavelength of 470 nm) yielded a linear response over a larger Delta T range due to the broader energy band and inherently smaller wavelength shift per unit energy in this spectral region. Based on the findings in this work, the desired MoS2 flake thickness to optimize sensitivity for thermal measurements is <30 nm. The method was validated by comparing the surface temperature rise measured from a MoS2 flake, using 470 and 675 nm probing wavelengths, and directly from the semiconductor channel of a GaN high electron mobility transistor, using a 365 nm probing wavelength. To demonstrate the viability for thermometry of ultrawide band gap (UWBG) semiconductor devices, the method was applied to perform 2D mapping of the temperature rise of the semiconductor channel of a UWBG beta-Ga2O3 heterostructure field effect transistor. Ultimately, this work provides a framework for employing the layered morphology and highly nonlinear exciton resonance of transition metal dichalcogenides to probe the local thermal environment and also provides an avenue for probing thickness-dependent and temperature-dependent excitonic characteristics of thin layered materials.

Topological insulators (TIs) are promising for efficient spin-orbit torque (SOT) switching of ferromagnets due to spin-momentum locking of their surface states. However, bulk-conducting channels limit their full potential for low-power operations. Here we synthesize bulk-insulating Bi2Se3 on (BiIn)2Se3/In2Se3 buffer layers by molecular beam epitaxy and compare their SOT efficiency to bulk-conducting Bi2Se3 using Kerr rotation and second harmonic Hall measurements. For bulk-insulating Bi2Se3, we find a 4-fold reduction in critical current density to switch an adjacent NiFe layer and a 5-10-fold enhancement in SOT efficiency, unambiguously demonstrating that Fermi level tuning can significantly enhance switching performance. This is attributed to current flowing predominantly through the top TI layer, where the generated spins are in close proximity to the NiFe interface. In addition, our heterostructures are grown in situ, where the clean interface can facilitate strong hybridization and the formation of "descendent states" with spin-momentum locking, leading to enhanced SOT efficiency.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe2) are attractive nanomaterials for quantum information applications due to single-photon emission (SPE) from intrinsic atomic defects. Defect and strain engineering techniques have been developed to produce high purity, deterministically placed SPE in WSe2. However, a major challenge in the application of these techniques is the low temperature required to observe defect-bound TMD exciton emission, typically limiting SPE to T < 30 K. SPE at higher temperatures either loses purity or requires integration into complex devices such as optical cavities. Here, 2D heterostructure engineering and molecular functionalization are combined to achieve high purity (>90%) SPE in strained WSe2 persisting to over T = 90 K. Covalent diazonium functionalization of graphite in layered WSe2/graphite heterostructures maintains high purity up to T = 90 K and single-photon source integrity up to T = 115 K. This method preserves the best qualities of SPE from WSe2 while increasing working temperature to more than three times the typical range. This work demonstrates the versatility of surface functionalization and heterostructure design to synergistically improve the properties of quantum emission and offers new insights into the phenomenon of SPE from 2D materials.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive nanomaterials for quantum information applications due to single photon emission (SPE) from atomic defects, primarily tungsten diselenide (WSe2) monolayers. Defect and strain engineering techniques have been developed to yield high purity, deterministically positioned SPE in WSe2. However, a major challenge in application of these techniques is the low temperature required to observe defect-bound TMD exciton emission, typically limiting SPE to T<30 K. SPE at higher temperatures either loses purity or requires integration into complex devices such as optical cavities. Here, 2D heterostructure engineering and molecular functionalization are combined to achieve high purity (>90%) SPE in strained WSe2 persisting to over T=90 K. Covalent diazonium functionalization of graphite in a layered WSe2/graphite heterostructure maintains high purity up to T=90 K and single-photon source integrity up to T=115 K. This method preserves the best qualities of SPE from WSe2 while increasing working temperature to more than three times the typical range. This work demonstrates the versatility of surface functionalization and heterostructure design to synergistically improve the properties of quantum emission and offers new insights into the phenomenon of SPE from 2D materials.

We report a voltage-induced degradation technique using conductive atomic force microscopy to enhance the single-photon purity and reproducibility of quantum emitters in monolayer tungsten diselenide (WSe2). By applying a controlled electric field across a monolayer WSe2/poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) on a silicon substrate, localized degradation is induced around nanoindented emitter sites in the WSe2. This process selectively suppresses defect-bound exciton emissions while preserving emission from pristine regions within the indentations. Photoluminescence and second-order correlation measurements at 18 K demonstrate a substantial increase in single-photon purity when comparing emitters from untreated and voltage-treated regions. Emitters from untreated regions showed average values of g2(0) near or above the 0.5 threshold. In contrast, emitters from voltage-treated regions exhibited g2(0) values consistently below 0.14, with most falling near 0.05, demonstrating high-purity single-photon emission well below the g2(0) < 0.5 threshold. Importantly, the voltage-induced degradation method significantly boosts the production yield of high-purity single-photon emitters with g2(0) < 0.2 in over 10% of treated sites─an order of magnitude improvement over typical yield in two-dimensional (2D) materials. This nonvolatile, spatially selective approach enhances both emitter purity and yield without compromising emission intensity, offering a scalable and reliable route for integrating high-quality quantum emitters into photonic platforms. Integration with spectral tuning strategies such as strain engineering, local dielectric patterning, or electrostatic gating could further enable deterministic, wavelength-selective single-photon sources for advanced quantum photonic applications.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS2. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS2 conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS2 conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides.

The van der Waals material hexagonal boron nitride (hBN) has emerged as a promising candidate for hosting room temperature single-photon emitters (SPEs) for next-generation quantum technologies. However, the requirement of a high temperature anneal (850 °C or higher) to activate the SPEs in hBN makes it difficult to integrate into hybrid structures that cannot tolerate such temperatures, including all silicon-based circuits. In this work, we present a method to deterministically activate quantum emitters in multilayered hBN on a process substrate, followed by a zero thermal budget transfer to a target substrate. This technique does not lead to any degradation or loss of photon purity in the hBN emitters and provides a procedure for combining high-purity emitters with other exciting photonic, magnetic, or electrical properties to explore new physical phenomena. The ability to transfer hBN emitters onto arbitrary substrates creates new technological possibilities to incorporate these quantum photonic properties into photonic integrated circuits and plasmonic devices.

Quantum photonics promises significant advances in secure communications, metrology, sensing, and information processing/computation. Single-photon sources are fundamental to this endeavor. However, the lack of high-quality single photon sources remains a significant obstacle. We present here a paradigm for the control of single photon emitters (SPEs) and single photon purity by integrating monolayer WS2 with the organic ferroelectric polymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). We demonstrate that the ferroelectric domains in the P(VDF-TrFE) film control the purity of single photon emission from the adjacent WS2. By switching the ferroelectric polarization, we reversibly tune the single photon purity between the semiclassical and quantum light regimes, with single photon purities as high as 94%. This demonstrates a method for modulating and encoding quantum photonic information, complementing more complex approaches. This multidimensional heterostructure introduces an approach for control of quantum emitters by combining the nonvolatile ferroic properties of a ferroelectric with the radiative properties of the zero-dimensional atomic-scale emitters embedded in the two-dimensional WS2 semiconductor monolayer.

Exciton-plasmon coupling between two-dimensional transition metal dichalcogenides and metallic nanostructures has attracted much attention as a means of creating room temperature polaritons and controlling the optoelectronic properties of these hybrid quasiparticles. Prior investigations of strained monolayer transition metal dichalcogenides, that host single photon emission, coupled to plasmonic nanostructures have remained in the weak coupling regime where the interaction is characterized by Purcell enhancement of emission rates. Here we achieve site-specific coupling between spatially localized excitons in nanoindented WSe2 and localized surface plasmon resonances hosted by Au nanodiscs, thereby producing mode splitting within the scattered light spectrum. This establishes nanoindentation as a means of tailoring the topography of two-dimensional materials around plasmonic resonators. By tuning the localized surface plasmon resonance via disc diameter, we observe an avoided crossing between the exciton and plasmon resonances with room temperature mode splitting of 78 +/- 4 meV. We describe the exciton-plasmon interaction with a self-consistent theoretical framework based on cavity electrodynamics and arrive at an exciton-plasmon coupling strength of 23 +/- 5 meV. The apparent discrepancy between the mode splitting and coupling strength arises from enhanced excitonic absorption, a phenomenon that dominates when the coupling strength is less than the line width and is responsible for larger than expected mode splitting in scattering spectra. Our observations are consistent with full-wave electromagnetic models and place the exciton-plasmon system within the intermediate coupling regime. Enhanced absorption plays a key role in shaping the scattering spectra from plasmonic platforms where the resonator is often much more lossy than the exciton and we therefore recommend that it should be accounted for when estimating the coupling strength based on scattering data from such systems. We also suggest pathways to increase the coupling strength for achieving strong coupling between plasmons and the strain localized excitons that contribute to single photon emission.

Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies, but creation and placement are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision using an atomic force microscope. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs. Because monolayer WSe2 is a direct gap semiconductor, SPE emission at a given wavelength is often intermixed with classical light, reducing the purity of the quantum emission. We show that this undesirable classical emission, arising primarily from defect bound excitonic processes, is significantly suppressed by electrostatic gating or incorporating the WSe2 layer in a simple van der Waals heterostructure, resulting in values of the autocorrelation function g(2)(t=0) as low as 0.07 at low temperature. In addition, the SPE intensity can be strongly modulated by changing the polarity of the gate bias, a feature of technological importance for practical applications.

Two-dimensional (2D) hexagonal boron nitride (hBN) is one of the most promising candidates to host solid-state single photon emitters (SPEs) for various quantum technologies. However, the 2D nature with an atomic-scale thickness leads to inevitable challenges in spectral variability caused by substrate disturbance, lattice strain heterogeneity, and defect variation. Here, three-dimensional (3D) nanoarchitectured hBN is reported with integrated SPEs from native defects generated during high-temperature chemical vapor deposition (CVD). The 3D hBN has a quasi-periodic gyroid minimal surface structure and is composed of a continuous 2D hBN sheet with built-in convex and concave curvatures that promote the formation of optically active and thermally robust native defects. The free-standing feature of the gyroid hBN with a nearly zero mean curvature can effectively eliminate the substrate disturbance and minimize lattice strain heterogeneity. As a result, naturally occurring defects with a narrow SPE spectral distribution can be created and activated as color centers in the 3D hBN, and the density of the SPEs can be tailored by CVD temperature.

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials [1]. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed". This leads to the formation of polaronic quasiparticles that dramatically impact charge transport, surface reactivity, thermoelectric and optical properties, as observed in a variety of crystals and interfaces composed of polar materials [2-6]. Similarly, when oscillations of the charge density couple to conduction electrons the more elusive plasmon polaron emerges [7], which has been detected in electron-doped semiconductors [8-10]. However, the exploration of polaronic effects on low energy excitations is still in its infancy in two-dimensional (2D) materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of SL WS$_2$. By using micro-focused angle-resolved photoemission spectroscopy (microARPES) during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the SL WS$_2$ conduction band minimum (CBM). Our results are explained by an effective many-body model in terms of a coupling between SL WS$_2$ conduction electrons and graphene plasmon modes. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides (TMDs) [11-15].

This chapter covers state-of-the-art development and results in tip-enhanced Raman spectroscopy (TERS) as a significant configuration of near-field nanospectroscopy. The chapter develops by drawing attention to the importance of TERS with a background motivation of the specific research area. Subsequently, the development of TERS instrumentation is discussed in detail, stretching its limit in terms of spatial resolution using atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to circumvent the diffraction limit. To understand various chemical processes precisely, TERS studies in high- and ultrahigh-vacuum (UHV) for AFM- and STM-assisted measurements are elaborated with up-to-date information, including the study of a single molecule. Finally, low-temperature UHV-STM is described for molecular as well as submolecular level measurements using TERS. In describing various applications of TERS, the chapter elaborates on studies of inorganic materials of strategic importance, e.g., strained Si and quantum dots, along with other layered MXenes and Van der Waal bonded materials. An important area of molecular switching is also reported for the chemical identification at the nanoscale using TERS. The study of biomolecules is the most challenging yet most fascinating application of TERS, which is used for both DNA and RNA sequencing, including the study of bacteria and viruses. TERS being chemically sensitive, is also used for the understanding catalytic properties of plasmonic, bimetallic, and organometallic phthalocyanine materials. Tip-enhanced fluorescence microscopy is specifically reported for studying fluid cracking catalysts. The nano-spectroscopic study is further extended to imaging 2D transition metal dichalcogenide stacking as near-field second-harmonic generation efficiency is greatly enhanced by excitons. Finally, the chapter concludes with its important application of tip-enhanced photoluminescence measurements for understanding excitonic properties of semiconductors as luminescence characteristics are essentially confined to a few nm or sub-nm region owing to compositional variation, presence of defects or impurities, and strain.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi2Se3 and exfoliated 2D metallic ferromagnet Fe3GeTe2 (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be J(c) approximate to 1.2 x 10(6) A/cm(2), the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi2Se3. From second harmonic Hall measurements, we further determine the SOT efficiency (xi(DL)) to be in the range of 1.8 +/- 0.3 and 1.4 +/- 0.08 between 5 and 150 K, comparable to the highest values reported for Bi2Se3. Our density functional theory calculations find that the weak interlayer interactions at the Bi2Se3/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi2Se3. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.

The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS 2 , systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS 2 /graphene stacks employing first-principles calculations and find that the lower conduction band of WS 2 at the K / K ′ valleys (the C B − band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at the Q point of WS 2 mediates the coupling between C B − and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

The ability to assemble layers of two-dimensional (2D) materials to form permutations of van der Waals heterostructures provides significant opportunities in materials design and synthesis. Interlayer interactions can enable desired properties and functionality, and understanding such interactions is essential to that end. Here we report formation of interlayer exciton-phonon bound states in Bi2Se3/WS2 heterostructures, where the Bi2Se3 A1(3) surface phonon, a mode particularly susceptible to electron-phonon coupling, is imprinted onto the excitonic emission of the WS2. The exciton-phonon bound state (or exciton-phonon quasiparticle) presents itself as evenly separated peaks superposed on the WS2 excitonic photoluminescence spectrum, whose periodic spacing corresponds to the A1(3) surface phonon energy. Low-temperature polarized Raman spectroscopy of Bi2Se3 reveals intense surface phonons and local symmetry breaking that allows the A1(3) surface phonon to manifest in otherwise forbidden scattering geometries. Our work advances knowledge of the complex interlayer van der Waals interactions and facilitates technologies that combine the distinctive transport and optical properties from separate materials into one device for possible spintronics, valleytronics, and quantum computing applications.

Second harmonic generation (SHG) is a nonlinear optical response arising exclusively from broken inversion symmetry in the electric-dipole limit. Recently, SHG has attracted widespread interest as a versatile and noninvasive tool for characterization of crystal symmetry and emerging ferroic or topological orders in quantum materials. However, conventional far-field optics is unable to probe local symmetry at the deep subwavelength scale. Here, we demonstrate near-field SHG imaging of 2D semiconductors and heterostructures with the spatial resolution down to 20 nm using a scattering-type nano-optical apparatus. We show that near-field SHG efficiency is greatly enhanced by excitons in atomically thin transition metal dichalcogenides. Furthermore, by correlating nonlinear and linear scattering-type nano-imaging, we resolve nanoscale variations of interlayer stacking order in bilayer WSe2, and reveal the stacking-tuned excitonic light-matter-interactions. Our work demonstrates nonlinear optical interrogation of crystal symmetry and structure-property relationships at the nanometer length scales relevant to emerging properties in quantum materials.

The interaction between accumulated spins on the surface of a heavy metal (HM) and the magnetization of an adjacent magnetic material leads to various spin phenomena, such as spin-orbit torque, spin pumping, and spin Hall magnetoresistance (SHMR). However, the exploration of device applications based on these spin phenomena is often limited by the low charge-to-spin conversion efficiency of the HM. Authors of recent studies have suggested that topological insulators (TIs) are promising candidates for device applications due to their potentially higher charge-to-spin conversion efficiency. Here, we report a multifaceted study of a bilayer structure consisting of Bi2Se3 and Y3Fe5O (YIG) and demonstrate an approach based on angle-dependent magnetoresistance (ADMR) measurements to determine the effective charge-to-spin conversion efficiency in TIs. Our ferromagnetic resonance measurements demonstrate efficient spin pumping from YIG to Bi2Se3, which is further confirmed by detection of an electromotive force generated in Bi2Se3 via spin-to-charge conversion. Our ADMR measurements show that the interfacial spin diffusion can significantly affect the charge transport in a way like the SHMR effect and provide an estimate of the charge-to-spin conversion efficiency in Bi2Se3 of similar to 0.1-0.4. Neglecting to account for the large out-of-plane magnetoresistance of the Bi2Se3 results in a fivefold overestimate of the charge-to-spin conversion efficiency.

We present a method utilizing an applied electrostatic potential for suppressing the broad defect bound excitonic emission in two-dimensional materials (2DMs) which otherwise inhibits the purity of strain induced single photon emitters (SPEs). Our heterostructure consists of a WSe2 monolayer on a polymer in which strain has been deterministically introduced via an atomic force microscope (AFM) tip. We show that by applying an electrostatic potential, the broad defect bound background is suppressed at cryogenic temperatures, resulting in a substantial improvement in single photon purity demonstrated by a 10-fold reduction of the correlation function g(2)(0) value from 0.73 to 0.07. In addition, we see a 2-fold increase in the intensity of the SPEs as well as the ability to activate/deactivate the emitters at certain wavelengths. Finally, we present an increase in the operating temperature of the SPE up to 110 K, a 50 K increase when compared with the results when no electrostatic potential is present.