Quantifying charge carrier concentration in ZnO thin films by Scanning Kelvin Probe Microscopy.

In the last years there has been a renewed interest for zinc oxide semiconductor, mainly triggered by its prospects in optoelectronic applications. In particular, zinc oxide thin films are being widely used for photovoltaic applications, in which the determination of the electrical conductivity is of great importance. Being an intrinsically doped material, the quantification of its doping concentration has always been challenging. Here we show how to probe the charge carrier density of zinc oxide thin films by Scanning Kelvin Probe Microscopy, a technique that allows measuring the contact potential difference between the tip and the sample surface with high spatial resolution. A simple electronic energy model is used for correlating the contact potential difference with the doping concentration in the material. Limitations of this technique are discussed in details and some experimental solutions are proposed. Two-dimensional doping concentration images acquired on radio frequency-sputtered intrinsic zinc oxide thin films with different thickness and deposited under different conditions are reported. We show that results inferred with this technique are in accordance with carrier concentration expected for zinc oxide thin films deposited under different conditions and obtained from resistivity and mobility measurements.

EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative.

We report a simple technique for mapping Electrostatic Force Microscopy (EFM) bias sweep data into 2D images. The method allows simultaneous probing, in the same scanning area, of the contact potential difference and the second derivative of the capacitance between tip and sample, along with the height information. The only required equipment consists of a microscope with lift-mode EFM capable of phase shift detection. We designate this approach as Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM). An open-source MATLAB Graphical User Interface (GUI) for images acquisition, processing and analysis has been developed. The technique is tested with Indium Tin Oxide (ITO) and with poly(3-hexylthiophene) (P3HT) nanowires for organic transistor applications.

Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution.

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip-sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip-sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip-sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip-sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.

Different sites and modes of tracer injection for mapping the sentinel lymph node in patients with breast cancer.

Several studies have been published describing the techniques of identification of the "sentinel lymph node" (SN). There are marked differences in the techniques used by different investigators. Although agreement exists about the tracer particle size and the volume of injection, it is unknown what is the best site where to inject the tracer or the vital dye. The aim of the present study was to define the influence of different sites of injection on imaging of the lymphatic ducts and their SNs. We performed a pilot study in 30 consecutive patients with breast cancer who underwent SN biopsy by means of radioguided surgery and vital blue dye mapping. The patients were divided into six groups of five patients each; each patient was given a subdermal (ID) or peritumoral (IP) injection of radiotracer (300 microCi in 150 mL of 99mTc-HSA microcolloids; Albures, Amersham Sorin) above the tumor site in order to localize the SN. After the identification of the SN, a second injection of radiotracer was performed, which was different in each patient subset. In some cases more than one lymph node appeared on the lymphoscintigraphic scans after the second injection of radiotracer. When the peritumoral route was used it took longer to visualize the lymphatic pathways. For the ID route, injection at the exact skin projection over the tumor is optimal. Internal mammary lymph nodes were identified by both IP (2) and ID (1) injection, irrespective of the quadrant in which the tracer was injected. Our findings support the hypothesis of a precise topographic correspondence between the primary tumor and its specific SN. The subdermal route is more accurate than the intraparenchymal route, as it allows faster identification of the lymphatic vessels and SN. We believe these observations should be taken into account for the proper selection of the injection site of either vital dye or radiopharmaceuticals.