Peter J. Pauzauskie

Photothermal heating remains a major challenge during the optical excitation of nanoscale quantum sensors [C. Bradac, S. F. Lim, H. C. Chang, I. Aharonovich, Adv. Opt. Mater. 8 , 2000183 (2020)]. At low laser powers, heating of the sensor leads to the thermal drift of quantum electronic transition energies. At high laser powers, sensors degrade due to thermally induced phase transitions, including melting. Here, we report a general approach for mitigating the photothermal heating of nanoscale quantum sensors through solid-state laser refrigeration of the sensors’ underlying substrate. Experimentally, both nanodiamonds and gold nanocrystals are attached separately to a ceramic laser-cooling substrate through van der Waals interaction. A combination of Debye–Waller factor thermometry and optically detected magnetic resonance spectroscopy of nitrogen vacancy centers is used to demonstrate that solid-state laser refrigeration of the substrate can reduce the temperature of attached nanodiamond payloads by 25 ± 3 K below steady-state operating temperatures. Stokes/anti-Stokes Raman spectroscopy is used to demonstrate reduced photothermal heating of plasmonic gold nanocrystals by 8.8 ± 2.7 K during surface-enhanced Raman spectroscopy sensing of rhodamine-6G molecules. These results suggest that solid-state laser cooling could open up experimental possibilities for rapid feedback-controlled cooling of a wide range of nanoscale quantum resources.

Ion interactions with charged surfaces are fundamental to electrochemical, geochemical and biological systems, yet the impact of charging on interfacial structure and dynamics is poorly understood. Here we investigate the adsorption and precipitation of multivalent ions on mica using molecularly resolved atomic force microscopy. Although divalent ions form continuous hydroxide monolayers in a manner consistent with classical models, trivalent ions adopt complex states associated with strong overcharging, including ordered ion networks, cluster arrays and microphase-separated films not predicted by those models. Monte Carlo simulations show that such states emerge from charge frustration arising when restrictions on repelling charges prevent the minimization of electrostatic forces. Due to their universal nature across cation types, the results provide general principles underlying charge-driven nanostructure formation and insights for using electric fields to direct materials synthesis.

Rigorous understanding of assembly in colloidal systems is crucial to the development of tailored nanostructured materials. Despite extensive studies, a mechanistic understanding of the dynamics governing encounters of colloidal particles remains an ongoing challenge. We study colloidal encounter dynamics by inducing assembly through optical tweezers that impose an external attractive field for cubic-phase sodium yttrium fluoride nanocrystals. We show that surface roughness of the nanocrystals is a decisive factor for contact leading to assembly between the nanocrystals, manifested by the roughness-dependent hydrodynamic resistivity. This provides direct evidence that dynamics are equally important to energetics in understanding assembly.

Oriented attachment (OA) occurs when nanoparticles in solution align their crystallographic axes prior to colliding and subsequently fuse into single crystals. Traditional colloidal theories such as DLVO provide a framework for evaluating OA but fail to capture key particle interactions due to the atomistic details of both the crystal structure and the interfacial solution structure. Using zinc oxide as a model system, we investigated the effect of the solvent on short-ranged and long-ranged particle interactions and the resulting OA mechanism. In situ TEM imaging showed that ZnO nanocrystals in toluene undergo long-range attraction comparable to 1kT at separations of 10 nm and 3kT near particle contact. These observations were rationalized by considering non-DLVO interactions, namely, dipole-dipole forces and torques between the polar ZnO nanocrystals. Langevin dynamics simulations showed stronger interactions in toluene compared to methanol solvents, consistent with the experimental results. Concurrently, we performed atomic force microscopy measurements using ZnO-coated probes for the short-ranged interaction. Our data are relevant to another type of non-DLVO interaction, namely, the repulsive solvation force. Specifically, the solvation force was stronger in water compared to ethanol and methanol, due to the stronger hydrogen bonding and denser packing of water molecules at the interface. Our results highlight the importance of non-DLVO forces in a general framework for understanding and predicting particle aggregation and attachment.

The dynamics, orientational anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO2), and lithium nitrate (LiNO3), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. The 2D IR, NMR, and viscosity results show that LiNO2 exhibits longer correlation times, lower diffusivity, and nearly 4 times greater viscosity compared to those of the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion network interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO2 and LiNO3 solutions show significantly weakened chemical interactions between the lithium cations and the methyl thiocyanate when compared with those of the lithium halide salts.