Solid–Liquid Phase Transitions In article number 2300542, Franklin Liou, Hsin-Zon Tsai, Alex Zettl, Johannes Lischner, Michael Crommie, and co-workers use scanning tunneling microscopy to image a solid–liquid phase transition with sub-nanometer spatial resolution at the surface of a graphene field-effect transistor. They use electrostatic gating to drive reversible transitions between a quasi-1D molecular solid and a 2D ionic liquid phase. Transiently heating and cooling the device enables non-equilibrium melting dynamics to be visualized.

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs <i>p-n</i> diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral <i>p-n</i> homojunction photovoltaic. By combining surface and edge contacts for <i>p-n</i> diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.

Two-dimensional (2D) transition metal chalcogenides (TMDs) have drawn significant attention in recent years due to their extraordinary optical and electronic properties. As heat transfer plays an important role in device performance, various methods such as optothermal Raman spectroscopy and time-domain thermoreflectance have been developed to measure the thermal conductivity and interfacial thermal conductance in 2D van der Waals (vdW) heterostructures. Here, we employ the vibrational-pump-visible-probe (VPVP) spectroscopy to directly visualize the heat transfer process in a heterostructure of multilayer h-BN and monolayer WS2. Following an impulsive vibrational excitation of h-BN in the mid-infrared, we probe the heat transfer from h-BN through WS2 and finally to the substrate from the subpicosecond to the submicrosecond timescale. The interfacial thermal conductance of the h-BN/WS2 and WS2/SiO2 interfaces is obtained by corroborating the experiments with heat transfer calculations based on the Fourier’s law of heat conduction. Our study demonstrates an alternative, time-resolved optical method to measure cross-plane heat dissipation and opens up a new pathway to investigate the interlayer electron–phonon and phonon–phonon interactions in vdW heterostructures.

Hetero-modulated neural activation is vital for adaptive information processing and learning that occurs in brain. To implement brain-inspired adaptive processing, previously various neurotransistors oriented for synaptic functions are extensively explored, however, the emulation of nonlinear neural activation and hetero-modulated behaviors are not possible due to the lack of threshold switching behavior in a conventional transistor structure. Here, a 2D van der Waals float gate transistor (FGT) that exhibits steep threshold switching behavior, and the emulation of hetero-modulated neuron functions (integrate-and-fire, sigmoid type activation) for adaptive sensory processing, are reported. Unlike conventional FGTs, the threshold switching behavior stems from impact ionization in channel and the coupled charge injection to float gate. When a threshold is met, a sub-30 mV dec^-1 increase of transistor conductance by more than four orders is triggered with a typical switch time of approximately milliseconds. Essentially, by feeding light sensing signal as the modulation input, it is demonstrated that two typical tasks that rely on adaptive neural activation, including collision avoidance and adaptive visual perception, can be realized. These results may shed light on the emulation of rich hetero-modulating behaviors in biological neurons and the realization of biomimetic neuromorphic processing at low hardware cost.

Abstract Employing the original, all-optical method, we quantify the magnetic susceptibility of a two-dimensional electron gas (2DEG) confined in the MoSe 2 monolayer in the range of low and moderate carrier densities. The impact of electron–electron interactions on the 2DEG magnetic susceptibility is found to be particularly strong in the limit of, studied in detail, low carrier densities. Following the existing models, we derive the value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>g</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.5</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.4</mml:mn> </mml:math> for the bare (in the absence of the interaction effects) g -factor of the ground state electronic band in the MoSe 2 monolayer. The derived value of this parameter is discussed in the context of estimations from other experimental approaches. Surprisingly, the conclusions drawn differ from theoretical ab initio studies.

Reliable methods to create quantum emitters in hexagonal boron nitride (hBN) are highly sought after for scalable applications in quantum photonic devices. Specifically, recent efforts have focused on defects in hBN with a zero phonon line at 2.8 eV (436 nm). Here, we employ carbon-doped hBN crystals that were irradiated by an electron beam to generate these emitters and perform annealing treatments to investigate the stability of the emitters. We find that the blue emitters are stable up to ∼800 °C. However, upon annealing to 1000 °C, the emitters disappear, and a family of other emitters appears in the region of hBN that had been irradiated by an electron beam. Our findings contribute to the understanding of emitter species and emitter formation in hBN.

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS₂ due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS₂ devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS₂/1T-TaS₂ heterostructures in which the number of endotaxial metallic H-TaS₂ monolayers dictates the number of resistance transitions in 1T-TaS₂ lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Non-negative matrix factorization (NMF) is widely applied to analyze pattern data in an unsupervised manner. It imposes hard non-negativity constraints on factors to extract intrinsic characteristics from an input matrix, though demanding complicated optimization techniques which hinder the general applicability. Toward flexible formulation, we propose weakly non-negative factorization. In contrast to the strict non-negative approach, our method permits factors to contain small amount of negative values. The relaxation theoretically leads to an efficient factorization formulation which can be implemented by means of off-the-shelf techniques used in a deep learning literature. Thus, the method is flexibly applicable to versatile factorization tasks, such as deep NMF and structured NMF. In the experiments on the NMF-related tasks, we demonstrate that the weak non-negativity produces effective factors similarly to NMF and the method exhibits favorable performance in comparison to the other approaches.

Moiré effects in vertical stacks of two-dimensional crystals give rise to new quantum materials with rich transport and optical phenomena that originate from modulations of atomic registries within moiré supercells. Due to finite elasticity, however, the superlattices can transform from moiré-type to periodically reconstructed patterns. Here we expand the notion of such nanoscale lattice reconstruction to the mesoscopic scale of laterally extended samples and demonstrate rich consequences in optical studies of excitons in MoSe₂-WSe₂ heterostructures with parallel and antiparallel alignments. Our results provide a unified perspective on moiré excitons in near-commensurate semiconductor heterostructures with small twist angles by identifying domains with exciton properties of distinct effective dimensionality, and establish mesoscopic reconstruction as a compelling feature of real samples and devices with inherent finite size effects and disorder. Generalized to stacks of other two-dimensional materials, this notion of mesoscale domain formation with emergent topological defects and percolation networks will instructively expand the understanding of fundamental electronic, optical and magnetic properties of van der Waals heterostructures.

Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has so far limited the degrees of tunability and the microscopic understanding of exciton transport. In this work we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with radiative decay mechanisms dominating over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases, and have crucial implications for studying emerging states of matter such as Bose-Einstein condensation and optoelectronic applications based on exciton propagation.

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm^-1, and the room temperature charge carrier mobilitiy is around 80 000 cm²/(Vs) at a carrier density 1 × 10¹²cm^-12. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

Interacting electrons in one dimension (1D) are governed by the Luttinger liquid (LL) theory in which excitations are fractionalized. Can a LL-like state emerge in a 2D system as a stable zero-temperature phase? This question is crucial in the study of non-Fermi liquids. A recent experiment identified twisted bilayer tungsten ditelluride (tWTe₂) as a 2D host of LL-like physics at a few kelvins. Here we report evidence for a 2D anisotropic LL state down to 50 mK, spontaneously formed in tWTe₂ with a twist angle of ~ 3^o. While the system is metallic-like and nearly isotropic above 2 K, a dramatically enhanced electronic anisotropy develops in the millikelvin regime. In the anisotropic phase, we observe characteristics of a 2D LL phase including a power-law across-wire conductance and a zero-bias dip in the along-wire differential resistance. Our results represent a step forward in the search for stable LL physics beyond 1D.

Quantum Hall (QH) edge channels propagating along the periphery of two-dimensional (2D) electron gases under perpendicular magnetic field are a major paradigm in physics. However, groundbreaking experiments that could use them in graphene are hampered by the conjecture that QH edge channels undergo a reconstruction with additional nontopological upstream modes. By performing scanning tunneling spectroscopy up to the edge of a graphene flake on hexagonal boron nitride, we show that QH edge channels are confined to a few magnetic lengths at the crystal edges. This implies that they are ideal 1D chiral channels defined by boundary conditions of vanishing electronic wave functions at the crystal edges, hence free of electrostatic reconstruction. We further evidence a uniform charge carrier density at the edges, incompatible with the existence of upstream modes. This work has profound implications for electron and heat transport experiments in graphene-based systems and other 2D crystalline materials.