Philippe Guyot-Sionnest

Slow Electron Cooling in Colloidal Quantum Dots

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of ∼0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

n-Type colloidal semiconductor nanocrystals

Colloidal semiconductor nanocrystals combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors. Their colour is highly controllable, a direct consequence of quantum confinement on the electronic states. Such nanocrystals are a form of ‘artificial atoms’ (ref. 4) that may find applications in optoelectronic systems such as light-emitting diodes and photovoltaic cells, or as components of future nanoelectronic devices. The ability to control the electron occupation (especially in n-type or p-type nanocrystals) is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices. But conventional doping by introducing impurity atoms has been unsuccessful so far: impurities tend to be expelled from the small crystalline cores (as observed for magnetic impurities), and thermal ionization of the impurities (which provides free carriers) is hindered by strong confinement. Here we report the fabrication of n-type nanocrystals using an electron transfer approach commonly employed in the field of conducting organic polymers. We find that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states.

Permanent dipole moment and charges in colloidal semiconductor quantum dots

The presence of a large permanent dipole moment has important implications on our understanding of nanocrystalline materials. We report the results of dielectric dispersion studies of CdSe, ZnSe, and metal nanocrystals. Due to the polar nature of the wurtzite lattice, a permanent dipole moment may be expected for CdSe nanocrystals. However, dielectric dispersion studies reveal a similar magnitude of the dipole moment, as well as its dependence on size, in zinc-blende ZnSe nanocrystals. These dipole moments may be intrinsic attributes to all nonmetal nanoparticles with surface localized charges. We show evidence for thermally induced charging of both semiconductor and metal nanocrystals and present a simple picture to describe the linear dependence of dipole moment on the size in semiconductor nanocrystals.

Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands.

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with approximately 6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of approximately 10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times approximately 30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.

Variable Range Hopping Conduction in Semiconductor Nanocrystal Solids

The temperature and electrical field dependent conductivity of n-type CdSe nanocrystal thin films is investigated. In the low electrical field regime, the conductivity follows sigma approximately exp([-(T(*)/T)(1/2)] in the temperature range 10<T<120 K. At high electrical field, the conductivity is strongly field dependent. At 4 K, the conductance increases by 8 orders of magnitude over one decade of bias. At a very high field, conductivity is temperature independent with sigma approximately exp([-(E(*)/E)(1/2)]. The complete behavior is very well described by variable range hopping with a Coulomb gap.

Trion Decay in Colloidal Quantum Dots

Using charged films of colloidal CdSe/CdS core/shell quantum dots of approximately 3.5 to 4.5 nm core diameters and 0.6 to 1.2 nm thick CdS shells, the radiative and nonradiative decay of the negatively charged exciton, the trion T-, are measured. The T- radiative rate is faster than the exciton by a factor of 2.2 +/- 0.4 and estimated at approximately 10 ns. The T- lifetime is approximately 0.7-1.5 ns for the samples measured and is longer than the biexciton lifetime by a factor or 7.5 +/- 1.7.

Electrical Transport in Colloidal Quantum Dot Films

In nanocrystal solids, the small density of states of quantum dots makes it difficult to achieve metallic conductivity without band-like transport. However, to achieve band-like transport, the energy scale of the disorder should be smaller than the coupling energy. This is unlikely with the present systems due to the size polydispersivity. Transport by hopping may nevertheless lead to an increased mobility with decreasing temperature for some temperature range, and such behavior at finite temperature is not proof of band-like conduction. To date, at low temperature, variable range hopping in semiconductor or weakly coupled metal nanocrystal solids dominates transport, as in disordered semiconductors.

Optical trapping and alignment of single gold nanorods by using plasmon resonances.

We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using laser light slightly detuned from their longitudinal plasmon mode. Detuning to the long-wavelength side of the resonance allows stable trapping for several minutes, with an exponential dependence of trapping time on laser power (consistent with a Kramers escape process). Detuning to the short-wavelength side causes repulsion of the rods from the laser focus. Alignment of the long axis of the rods with the trapping laser polarization is observed as a suppression of rotational diffusion about the short axis.

Self-assembled molecular rectifiers

Self-assembled monolayers (SAMs) of C6H5–C≡C–C6H4–C≡C–C6H4SH (1) on Au(111) and Ag(111) exhibit electrical rectifying behavior when probed by the scanning tunneling microscope whereas SAMs derived from decanethiol, benzenethiol and 4-phenylethynylbenzenethiol do not. We suggest that rectification arises from the conjugated length of the molecular framework of 1 and an increase of the donor character of the sulfur atom to this system upon adsorption to the metal. This is supported by surface second-harmonic generation measurements which show a significantly larger second order optical nonlinearity of SAMs of 1 compared to the other systems.

Synthesis of colloidal HgTe quantum dots for narrow mid-IR emission and detection.

HgTe colloidal quantum dots are prepared via a simple two-step injection method. Absorption and photodetection with sharp edges, as well as narrow photoluminescence, are tunable across the near and mid-IR between 1.3 and 5 μm.