Theodosia Gougousi

Transient absorption (TA) measurements on vibrating metal nanoparticles suspended in solution offer a noncontact method for monitoring solid–liquid interfacial phenomena on the nanoscale and at ultrafast time scales. Here, we use this method to investigate the effect of capping molecules on the liquid slip at the surface of gold nanoparticles. In particular, we functionalize both bipyramidal and spherical gold nanoparticles with a series of capping molecules and use TA measurements to monitor the oscillations in plasmon resonance frequency as a function of time after excitation by a pump laser pulse. Analysis of the Fourier transform of these oscillations and comparison to theory reveal slip lengths that vary by nearly a factor of 3 for different capping molecules, even though the molecules have similar chemical end groups. Moreover, there is no clear relationship between the measured slip length and macroscopic measurements of surface wettability. The results indicate that nanoscale slip at high frequencies might depend more subtly on the detailed molecular arrangement between ligand and solvent molecules.

This study investigates the presence of titanium oxynitride bonds in titanium dioxide (TiO₂) thin films grown by atomic layer deposition (ALD) using tetrakis dimethyl amino titanium (TDMAT) and water at temperatures between 150 and 350 °C and its effect on the films’ optical and electrical properties. Compositional analysis using X‐ray photoelectron spectroscopy (XPS) reveals increased incorporation of oxynitride bonds as the process temperature increases. Furthermore, depth profile data demonstrates an increase in the abundance of this type of bonding from the surface to the bulk of the films. Ultraviolet‐visible spectroscopy (UV‐vis) measurements correlate increased visible light absorption for the films with elevated oxynitride incorporation. The optical constants (n, k) of the films show a pronounced dependence on the process temperature that is mirrored in the film conductivity. The detection of oxynitride bonding suggests a secondary reaction pathway in this well‐established ALD process chemistry, that may impact film properties. These findings indicate that the choice of process chemistry and conditions can be used to optimize film properties for optoelectronic applications.

Typical titanium oxide (TiO₂) films are transparent in the visible range, allowing for their index of refraction and thickness to be extracted by single-angle spectroscopic ellipsometry (SE) using a Cauchy model. However, TiO₂ films grown by atomic layer deposition (ALD) from tetrakis(dimethylamino)titanium (IV) (TDMAT) and H₂O at 350 °C absorb in the visible range due to the formation of Ti-O-N/Ti-N bonds. Single-angle SE is inadequate for extracting the optical constants of these films, as there are more unknowns (n, k, d) than measurable parameters (ψ, Δ). To overcome this limitation, we combined SE with transmission (T) measurements, a method known as SE + T. In the process, we developed an approach to prevent backside deposition on quartz substrates during ALD deposition. When applying a B-spline model to SE + T data, the film thicknesses on the quartz substrates closely matched those on companion Si samples measured via standard lithography. The resulting optical constants indicate a reduced refractive index, n, and increased extinction coefficient, k, when compared to purer TiO₂ thin films deposited via a physical vapor deposition (PVD) method, reflecting the influence of nitrogen incorporation on the optical properties.

A tunable nanotip optical cavity controls hybrid quantum states of a single emitter at room temperature.

Nonlinear index values as high as 1±.1x10^-9 cm²/W were measured of ALD based TiO₂ films. The 1000-fold increase in the index is believed to be due to the incorporation of nitrogen during growth.

We use a configurable plasmonic nano-cavity with a nanoscopic mode volume to achieve and control strong coupling to a single quantum dot under ambient conditions using tip-enhanced photoluminescence spectroscopy.

In this work, the authors compare the surface coverage obtained in the initial and intermediate steps of Al2O3 and TiO2 atomic layer deposition on MoS2 surfaces prepared by either mechanical exfoliation or chemical vapor deposition (CVD). They find that the film surface coverage is highly dependent on the origin and preparation of the MoS2 starting surface. While mechanical exfoliation can be used to quickly obtain few-layer MoS2 surfaces, these surfaces exhibit significant variability in quality and cleanliness, leading to unpredictable film growth. Conversely, MoS2 grown by CVD provides a more reliable starting surface resulting in significantly improved scattering in the surface coverage. They find that, on CVD MoS2, neither Al2O3 nor TiO2 film growth strongly exhibits temperature dependence described by the Langmuir adsorption model. For both processes, films up to 6 nm thick deposited on CVD MoS2 are not fully coalesced and exhibit a large concentration of pinhole type features.

Thermally managed Z-scan performed on ALD grown TiO₂ films demonstrated n₂ values of 1.7×10⁻¹¹ and 1.94 × 10⁻¹⁰cm²/W for films grown at 100°C and 250°C, respectively – greater than 1000X that of other growth methods.