Electron Microscopy Observation of TiO₂ Nanocrystal Evolution in High-Temperature Atomic Layer Deposition

Understanding the evolution of amorphous and crystalline phases during atomic layer deposition (ALD) is essential for creating high quality dielectrics, multifunctional films/coatings, and predictable surface functionalization. Through comprehensive atomistic electron microscopy study of ALD TiO₂ nanostructures at designed growth cycles, we revealed the transformation process and sequence of atom arrangement during TiO₂ ALD growth. Evolution of TiO₂ nanostructures in ALD was found following a path from amorphous layers to amorphous particles to metastable crystallites and ultimately to stable crystalline forms. Such a phase evolution is a manifestation of the Ostwald–Lussac Law, which governs the advent sequence and amount ratio of different phases in high-temperature TiO₂ ALD nanostructures. The amorphous–crystalline mixture also enables a unique anisotropic crystal growth behavior at high temperature forming TiO₂ nanorods via the principle of vapor-phase oriented attachment

Probing the Optical Properties and Strain-Tuning of Ultrathin Mo_1–<i>x</i>W_<i>x</i>Te₂

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo_0.91W_0.09Te₂, a semiconducting alloy of MoTe₂, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo_0.91W_0.09Te₂ transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe₂ counterpart. We apply tensile strain, for the first time, to monolayer MoTe₂ and Mo_0.91W_0.09Te₂ to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton–phonon intervalley scattering. Strained MoTe₂ and Mo_0.91W_0.09Te₂ extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics

Characterization of Few-Layer 1T′ MoTe₂ by Polarization-Resolved Second Harmonic Generation and Raman Scattering

We study the crystal symmetry of few-layer 1T′ MoTe₂ using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe₂ is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe₂ and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation

Nanoscale Heterogeneities in Monolayer MoSe₂ Revealed by Correlated Scanning Probe Microscopy and Tip-Enhanced Raman Spectroscopy

Understanding growth, grain boundaries (GBs), and defects of emerging two-dimensional (2D) materials is key to enabling their future applications. For quick, nondestructive metrology, many studies rely on confocal Raman spectroscopy, the spatial resolution of which is constrained by the diffraction limit (∼0.5 μm). Here we use tip-enhanced Raman spectroscopy (TERS) for the first time on synthetic MoSe₂ monolayers, combining it with other scanning probe microscopy (SPM) techniques, all with sub-20 nm spatial resolution. We uncover strong nanoscale heterogeneities in the Raman spectra of MoSe₂ transferred to gold substrates [one near 240 cm^–1 (A₁′), and others near 287 cm^–1 (E′), 340 cm^–1, and 995 cm^–1], which are not observable with common confocal techniques and appear to imply the presence of nanoscale domains of MoO₃. We also observe strong tip-enhanced photoluminescence (TEPL), with a signal nearly an order of magnitude greater than the far-field PL. Combining TERS with other SPM techniques, we find that GBs can cut into larger domains of MoSe₂, and that carrier densities are higher at GBs than away from them

Electrolyte Stability Determines Scaling Limits for Solid-State 3D Li Ion Batteries

Rechargeable, all-solid-state Li ion batteries (LIBs) with high specific capacity and small footprint are highly desirable to power an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. A variety of three-dimensional (3D) LIB architectures that maximize areal energy density has been proposed to address this need. The success of all of these designs depends on an ultrathin, conformal electrolyte layer to electrically isolate the anode and cathode while allowing Li ions to pass through. However, we find that a substantial reduction in the electrolyte thickness, into the nanometer regime, can lead to rapid self-discharge of the battery even when the electrolyte layer is conformal and pinhole free. We demonstrate this by fabricating individual, solid-state nanowire core–multishell LIBs (NWLIBs) and cycling these inside a transmission electron microscope. For nanobatteries with the thinnest electrolyte, ≈110 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. With electrolyte thickness increased to 180 nm, the self-discharge rate is reduced substantially, and the NWLIBs maintain a potential above 2 V for over 2 h. Analysis of the nanobatteries’ electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes directly through the electrolyte. Our study illustrates that, at these nanoscale dimensions, the increased electric field can lead to large electronic current in the electrolyte, effectively shorting the battery. The scaling of this phenomenon provides useful guidelines for the future design of 3D LIBs

Rydberg Excitons and Trions in Monolayer MoTe₂

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS2, MoSe2, WS2, and WSe2), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe2). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe2 to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton–phonon coupling. Furthermore, we observe a strongly gate-tunable exciton–trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe–Salpeter equation approach calculations. Our results help bring monolayer MoTe2 closer to its potential applications in near-infrared optoelectronics and photonic devices