Formation of PbSe/CdSe Core/Shell Nanocrystals for Stable Near-Infrared High Photoluminescence Emission

PbSe/CdSe core/shell nanocrystals with quantum yield of 70% were obtained by the “successive ion layer adsorption and reaction” technology in solution. The thickness of the CdSe shell was exactly controlled. A series of spectral red shifts with the CdSe shell growth were observed, which was attributed to the combined effect of the surface polarization and the expansion of carriers’ wavefunctions. The stability of PbSe nanocrystals was tremendously improved with CdSe shells

Polarised Photoluminescence from Surface-Passivated PbS Nanocrystals

Effective surface-passivation of PbS nanocrystals in aqueous colloidal solution has been achieved following treatment with CdS precursors. The resultant photoluminescent emission displays two distinct components, one originating from the absorption band-edge and the other from above the absorption band-edge. We show that both of these components are strongly polarised but display distinctly different behaviours. The polarisation arising from the band-edge shows little dependence on the excitation energy while the polarisation of the above-band-edge component is strongly dependent on the excitation energy. In addition, time resolved polarisation spectroscopy reveals that the above-band-edge polarisation is restricted to the first couple of nanoseconds, while the band-edge polarisation is nearly constant over hundreds of nanoseconds. We recognise an incompatibility between the two different polarisation behaviours, which enables us to identify two distinct types of surface-passivated PbS nanocrystal.Comment: Preprint, 19 pages, 4 figure

Dual Exposure Glass Layer Suspended Structures: A Simplified Fabrication Process for Suspended Nanostructures on Planar Substrates

We have developed and demonstrated here a simplified flexible fabrication process for glass nanomechanical systems. This process uses a single layer of spin on glass (SOG) material with two negative tone electron beam exposures at two different exposure energies to define the suspended and support structures, respectively. After development the SOG can be converted into glass. The process is additive and can be applied to any flat substrate. We have fabricated a variety of glass nanomechanical oscillators and measured their mechanical resonances using a mechanical piezoelectric driving force and optical interferometric detection. Suspended structures were fabricated with thickness of less than 50 nm and lateral dimensions of less than 100 nm supported anywhere from 150 to 800 nm above the substrate. Resonance frequencies for glass wires with both ends fixed (cross section 110 nm×180 nm) and lengths of 4–9 μm range from 7 to 30 MHz, with quality (Q) factors of over 1000. Annealing the structures in an oxygen ambient roughly doubles both the frequencies and the Q factors

Charge Induced Pattern Distortion in Low Energy Electron Beam Lithography

Charge induced pattern distortions in low voltage electron beam lithography in the energy range of 1 to 5 kV were investigated. Pattern distortion on conducting substrates such as silicon was found to be small, while significant pattern placement errors and pattern distortions were observed in the case of electrically insulating substrates caused by charge trapping and deflection of the incident electron beam. The nature and magnitude of pattern distortions were found to be influenced by the incident electron energy, pattern size, electrical conductivity, and secondary electron emission coefficient of the substrate. Theoretical modeling predicts the electron beam deflection to be directly proportional to the trapped surface charge density and inversely proportional to the accelerating voltage

Shape and Temperature Dependence of Hot Carrier Relaxation Dynamics in Spherical and Elongated CdSe Quantum Dots

Time-domain nonadiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in a CdSe spherical quantum dot (QD) and an elongated quantum dot (EQD) with the same diameter. The band gap is smaller, and the electron and hole states are denser in the EQD than in the QD. Also, the band gap shows a stronger negative temperature dependence in the EQD than in the QD. Higher frequency phonons are excited and scattered with electrons at higher temperatures for both QD and EQD. The electron-phonon coupling is generally stronger in the EQD than in the QD. The hot electron decay rates calculated from nonadiabatic molecular dynamics show a weaker temperature dependence than the T(-1) trend in both QD and EQD, which is attributed to the thermal expansion effect. Furthermore, the relaxation of hot electrons proceeds faster and shows stronger temperature dependence in the EQD than in the QD. Our work demonstrates that the shape of quantum dots has a strong impact on the electron decay dynamics

Origin of the Large Homogeneous Line Widths Obtained from Strongly Quantum Confined PbS nanocrystals at Room Temperature

We study the temperature dependence of the emission spectrum obtained from a colloidal suspension of PbS nanocrystals with an average diameter of 2.5 nm. The homogeneous component of the spectrum is obtained by employing a deconvolution procedure, with the aid of size selective excitation spectroscopy. The homogeneous line shape is found to be strongly temperature dependent, which is attributed to acoustic phonon broadening. At room temperature, an ultrafast exciton dephasing time of 5 fs is determined from the homogeneous line width, in agreement with a recent prediction. A large temperature-dependent shift of the peak position is also observed. The temperature dependence of the peak shift has the opposite sign to that observed in bulk PbS. This is directly attributed to increased acoustic phonon coupling via enhancement of the deformation potential in strongly quantum confined semiconductors. Both the emission peak shift and large temperature-dependent homogeneous line shape independently point to significantly enhanced acoustic phonon coupling in small, strongly quantum confined, PbS nanocrystals. This conclusion is in agreement with some long-standing theories