In the exploration of the nano-world of semiconductors there is a strong focus on low-dimensional structures and ultra-small devices. Two fundamental problems, which challenge progress in this field are: (i) large ensembles of nano-objects, like Quantum Dots (QDs), do not have identical geometrical shapes and electronic properties, and, (ii) the properties of a low-dimensional structure can be dominated by a few impurity atoms, whereas the properties of a macroscopic structure is determined by the quasi-continuous background of dopant impurities. To allow QDs and discrete impurities to be studied, novel experimental techniques are required. In this paper we describe how local luminescence has been excited from single QDs using electrons injected from a Scanning Electron Microscope (SEM), from the tip of a Scanning Tunneling Microscope (STM) or using highly focused photons for excitation. We present images of QDs as well as characteristic spectra of individual QDs. We finally show how the local character of the excitation enables us to excite and image individual impurities in low-dimensional structures, including the measurement of characteristic emission spectra from a single impurity atom in GaAs.

We describe the application of t€chniques for nano{ptical studies of quantum dots fabrricated via the Stranski-Krastanotr growth mode by MOVPE. The first method is p-PL in which excitation and detection is spatialy resrricted ro 0.1 I Fm by masking of the sample. In the s€cord technique we inject tocally low-energy carrien from an STM tip into *r9 semiconductor. These two rechniques share the ability to pedorm spgcEoscopic investigadons of single quantum do6, including excitation spectroscopies with energy selection either by the errrgy of the exciting light or by tlle potential applied between fte STM tip and the sample.

A liquid-cell-based cantilever sensor system operating in the dynamic mode has been developed and characterized, and the frequency spectra of commercial micrometre-scale silicon cantilevers in aqueous solutions have been studied. We report data demonstrating measurements of the resonance frequency shift induced upon phospholipid vesicle adsorption on an oscillating cantilever immersed in a liquid. A resonance frequency shift corresponding to an added mass of 450 pg has been measured, which is in good agreement with the estimated mass of 400 pg. In addition, the effect of varying the liquid volume in the cell on the frequency response has been investigated.

Silane micropatterns with motor proteins for creation of spatiotemporal chemical gradients on a chip

We have previously shown that selective heavy meromyosin (HMM) adsorption to predefined regions of nanostructured polymer resist surfaces may be used to produce a nanostructured in vitro motility assay. However, actomyosin function was of lower quality than on conventional nitrocellulose films. We have therefore studied actomyosin function on differently derivatized glass surfaces with the aim to find a substitute for the polymer resists. We have found that surfaces derivatized with trimethylchlorosilane (TMCS) were superior to all other surfaces tested, including nitrocellulose. High-quality actin filament motility was observed up to 6 days after incubation with HMM and the fraction of motile actin filaments and the velocity of smooth sliding were generally higher on TMCS than on nitrocellulose. The actomyosin function on TMCS-derivatized glass and nitrocellulose is considered in relation to roughness and hydrophobicity of these surfaces. The results suggest that TMCS is an ideal substitute for polymer resists in the nanostructured in vitro motility assay. Furthermore, TMCS derivatized glass also seems to offer several advantages over nitrocellulose for HMM adsorption in the ordinary in vitro motility assay.

A long-standing problem for piezo-electric inertial motors is the decrease of efficiency and step length at lower temperatures.We have developed a piezoelectric vibration driver for positioning in a temperature range of 4 - 400K for Scanning Probe Microscopes (SPM). The vibrator consists of two parallel piezoceramic plates, which are fixed rigidly to the support at their centre, and to the endpieces at their ends. The vibrator is symmetrical to the three perpendicular symmetric axes.There is no considerable decrease of steps size in the novel piezoresonance device while cooling to a low temperature, because the decrease of piezomodule d(31) is compensated by an increase of the mechanical efficiency factor Q of the vibrator at low temperatures.The described vibrator can shift linear rails, rotate and/or shift cylindrical bodies.

We have investigated a simple yet powerful method for making sharp scanning tunneling microscopy tips of Pt/Ir. It consists of three electrochemical polish/etch steps, not requiring any special micropolishing. The tips, as seen by high-resolution transmission electron microscopy, are sharper than 20 Å. Furthermore, they are smooth, without mini tips and covered with an oxide layer not much thicker than the native one (as seen from a freshly cut wire) when all three steps are applied.

Three cryostats were designed and constructed where,the cooling is achieved without boiling in the main vessel: 1) smooth controlled raising of pressure of the cryoliquid inside the main vessel with SPM; 2) using the cooling cell where the cryoliquid boils at a temperature a little lower than that of the main vessel; 3) using a porous ceramic cryovessel for cooling the liquid by filtration and evaporation it through the 4 - 6 mm thick walls of the vessel.

A quiet state of cryoliquid is of extreme importance for sensitive optical and scanning probe microscope investigations. A special cryostat was designed and constructed for preventing bubbling of cryoliquid by cooling the liquid diffusing it through the walls of porous material.

We have measured photon emission from individual luminescent states in GaInP/InP heterostructures, containing InP dots, using local injection from a scanning tunneling microscope tip. By changing the tip-sample bias we are able to see the Stark shift of the emission peaks, as well as the onset of impact ionization. We find that the exciton diffusion length is about 1 μm, while the minority carrier diffusion length is much less in our samples. Below the threshold for impact ionization the excitation is extremely local, limiting the excitation to one or a few quantum dots.

The ability to fabricate quantum dots using the Stranski-Krastanow growth technique has improved dramatically during the last number of years. Due to the large number of dots formed (typically 108–109 cm−3) the emission linewidth is inhomogeneously broadened. We have grown quantum dots of InP in between barriers of GaInP, as well as on top of GaInP, having a low density of dots in order to perform single-dot spectroscopy. These dots have been studied by photoluminescence and scanning tunneling luminescence. We find in photoluminescence that the fully formed dots have well-defined, sharp (0.04–1 meV) emission lines, which are very similar from dot to dot. In scanning tunneling luminescence we find that we can very locally excite only a few partially formed dots which have sharp emission lines (0.1 meV at 77 K). These emission lines display a quantum confined Stark effect when the applied tip to sample bias is varied. We can carefully determine the onset of exciton formation as a function of applied bias. The applied bias which is necessary for impact ionization is found to agree well with simple theory.

InP quantum dots embedded in Ga0.5In0.5P are investigated by injection luminescence. By using a masking technique we have improved the spatial resolution. At 77 K, the luminescence peak of the fully formed InP dots occurs at about 1.62 eV. In addition, in the 1.7–1.8 eV energy range, we observe a rich structure in the spectra with several sharp lines typically 3 meV in width. The origin of this luminescence is attributed to the partially formed InP quantum dots. This injection luminescence band also exhibits spatial variations both in the envelope as well as in the fine structure.