We use micro-Raman spectroscopy to measure the vibrational structure of the atomically precise cadmium selenide quantum dots CdSeXL, CdSeXL, and CdSeXL. These quantum dots have benzoate (X) and n-butylamine (L) ligands and tetrahedral (T) shape with edges that range from 1.7 to 2.6 nm in length. Investigating this previously unexplored size regime allows us to identify the transition from molecular vibrations to bulk phonons in cadmium selenide quantum dots for the first time. Room-temperature Raman spectra have broad CdSe peaks at 175 and 200 cm. Density functional theory calculations assign these peaks to molecular surface and interior vibrational modes, respectively, and show that the interior, surface, and ligand atom motion is strongly coupled. The interior peak intensity increases relative to the surface peak as the cluster size increases due to the relative increase in the polarizability of interior modes with quantum dot size. The Raman spectra do not change with temperature for molecular CdSeXL, while the interior peak narrows and shifts to higher energy as temperature decreases for CdSeXL, a spectral evolution typical of a phonon. This result shows that the single bulk unit cell contained within CdSeXL is sufficient to apply a phonon confinement model, and that CdSeXL, with its 2.1 nm edge length, marks the boundary between molecular vibrations and phonons.

F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is used widely as a hole-doping agent in photoresponsive organic semiconducting materials, yet relatively little is known about the photoresponses of the F4TCNQ·– anion generated via doping. Furthermore, there is still relatively little systematic exploration of how the properties of the local material or chemical environment impacts the driving force for generating these charge-transfer complexes. Here we present spectroscopic and photophysical studies of F4TCNQ in charge-transfer complexes (CTCs) with the electron donor N,N′-diphenyl-N-N′-di-p-tolylbenzene-1,4-diamine (MPDA) both in dichloroethane solution and polystyrene matrices. Integer charge transfer (ICT) between donor and acceptor occurs readily in dichloroethane solvent to form F4TCNQ·–:MPDA+ CTCs, due to a ∼150 mV difference in MPDA+/MPDA and F4TCNQ/F4TCNQ·– reduction potentials. Ultrafast spectroscopic studies of the CTC as well as electrochemically generated F4TCNQ·– and MDPA+ reveal that the photoresponses of these CTCs are dominated by that of the dopant anion, including rapid deactivation (800 fs) after excitation to the anion D1 excited state, followed by slower (∼10 ps) vibrational cooling in the anion D0 state. Excitation to the higher-lying D2 state results in a rapid relaxation to the D1 state, in contrast to direct D2 → D0 relaxation previously observed for F4TCNQ·– in the gas phase. CTCs embedded in polystyrene (PS) matrices are observed to lose their integer charge-transfer character upon evaporation of solvent, as evidenced by changes to electronic and vibrational absorption features associated with F4TCNQ·–. This change is attributed to the loss of solvent stabilization of the ion pair formed through the charge-transfer reaction. Ultrafast spectral measurements reveal that the photoresponses of the partial charge-transfer (PCT) species embedded in PS are still highly similar to those of the ICT species and unlike that of neutral F4TCNQ, implying the electronic properties of the PCT state are likewise dominated by properties of the reduced acceptor molecule. We conclude that excitation of ICT or PCT states introduces optical losses for photoresponses of doped organic semiconductor materials due to the large anion absorption cross section and its rapid, dissipative deactivation dynamics.

Plasmonic aluminum nanoparticles have emerged as an exciting new materials platform due to the high natural abundance of aluminum, their ability to be synthesized in the solution phase, and the potential of these materials to be used for photocatalysis and sensing. However, the photothermal properties of solution-processed aluminum nanoparticles, in particular, how phonon energy transfer depends on the particle size and surface properties, are critical for practical applications and are currently unexplored. Here we use transient absorption spectroscopy, in combination with simulations of phonon and thermal energy dissipation, to investigate the photoresponses of aluminum nanoparticles of various diameters (54, 85, 121, and 144 nm) suspended in 2-propanol. Fast thermal-transfer rates to the solvent (170–280 ps) are observed for particles of all sizes and are facilitated by native oxide coverage, as verified by a two-interface thermal energy-transfer model. Size-dependent phonon “breathing”/vibrational modes are also observed as oscillations in the total cross-section. We find that both the oscillation frequency and the damping rate increase as the diameter of the particles decreases. On the basis of the results of finite element calculations, we attribute the damping strength and oscillation period observed to a combination of the noncrystalline nature of the native oxide shell and the presence of surface-bound ligands, both of which increase the vibrational mode damping rates relative to bare Al and Al particles with a bare crystalline oxide shell. These insights should guide future work on controlling energy transfer through the use of size and surface tuning in sustainable aluminum nanomaterial systems for applications in catalysis and sensing.

We use micro-Raman spectroscopy to investigate how solvation affects chemical hole doping of single-layer graphene by iodine. Measuring the graphene hole density as a function of iodine concentration in cyclohexane, benzene, and water generates solvent-dependent isotherms. Fitting these data to Langmuir isotherms provides equilibrium constants of adsorption and maximum hole densities. Raman spectra of bilayer graphene in water at intermediate iodine concentrations reveal a split in the graphene G peak, indicating asymmetric doping. This result shows that discrepancies between the Langmuir fits and the data are explained by different adsorption thermodynamics on the top and bottom graphene surfaces. The equilibrium constant is largest in water and equal for benzene and cyclohexane. In contrast, the maximum hole density decreases from water to cyclohexane to benzene. Equilibrium constants in all solvents and the maximum hole density in water are explained by solvent polarity. More hole doping occurs in cyclohexane than in benzene, and density functional theory calculations suggest a mechanism where this result is caused by solvent modulation of the energies and shapes of donor and acceptor orbitals, rather than by competitive charge transfer with the solvent.