Combined Photo- and Thermionic Electron Emission from Low Work Function Diamond Films

abstract: In this dissertation, combined photo-induced and thermionic electron emission from low work function diamond films is studied through low energy electron spectroscopy analysis and other associated techniques. Nitrogen-doped, hydrogen-terminated diamond films prepared by the microwave plasma chemical vapor deposition method have been the most focused material. The theme of this research is represented by four interrelated issues. (1) An in-depth study describes combined photo-induced and thermionic emission from nitrogen-doped diamond films on molybdenum substrates, which were illuminated with visible light photons, and the electron emission spectra were recorded as a function of temperature. The diamond films displayed significant emissivity with a low work function of ~ 1.5 eV. The results indicate that these diamond emitters can be applied in combined solar and thermal energy conversion. (2) The nitrogen-doped diamond was further investigated to understand the physical mechanism and material-related properties that enable the combined electron emission. Through analysis of the spectroscopy, optical absorbance and photoelectron microscopy results from sample sets prepared with different configurations, it was deduced that the photo-induced electron generation involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. (3) Based on results from the first two studies, possible photon-enhanced thermionic emission was examined from nitrogen-doped diamond films deposited on silicon substrates, which could provide the basis for a novel approach for concentrated solar energy conversion. A significant increase of emission intensity was observed at elevated temperatures, which was analyzed using computer-based modeling and a combination of different emission mechanisms. (4) In addition, the electronic structure of vanadium-oxide-terminated diamond surfaces was studied through in-situ photoemission spectroscopy. Thin layers of vanadium were deposited on oxygen-terminated diamond surfaces which led to oxide formation. After thermal annealing, a negative electron affinity was found on boron-doped diamond, while a positive electron affinity was found on nitrogen-doped diamond. A model based on the barrier at the diamond-oxide interface was employed to analyze the results. Based on results of this dissertation, applications of diamond-based energy conversion devices for combined solar- and thermal energy conversion are proposed.Dissertation/ThesisPh.D. Physics 201

Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

International audienceHighly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions

Thermionic emission properties of novel carbon nanostructures.

Materials with low work function values (\u3c 2 eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the study of the thermionic emission of the hybrid structure phosphorus, (P) doped diamond nano crystals grown on conical carbon nanotubes (CCNTs). The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals. In the first part of this thesis thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7 eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. In the next part of the thesis we present work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in work function value of 2.23 eV. The reduced work-function is interpreted as due to the presence of the surface states and midband-gap states and no evidence for negative electron affinity was seen. However, Ultraviolet photo-spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~1.8 eV with a negative electron affinity (NEA) value of 1.2 eV. Detailed band diagrams are presented to support the observed values for both cases. In addition we determined the work function values of nanocrystalline P doped diamond films grown on W foil to be significantly lower, 1.0- 1.33 eV compared to the hybrid structure and polycrystalline film on Si substrates. We studied tungsten (W) nanowires as an alternative material in place of CCNT as the supporting and conducting channel for P doped diamond crystals in a new hybrid structure. We described the process of fabrication of arrays of vertical W nanowires by microwave plasma treatment and synthesis of P doped nanocrystalline diamond on top of the reduced W nanowires. Thermionic emission measurements from the alternative hybrid structure resulted in high value of the work function ~ 5.1 eV

The Effect of Temperature on the Electrical and Optical Properties of p-type GaN

The development of gallium nitride (GaN) light emitting devices has reached extraordinary echelons. As such, the characterization and analysis of the behavior of GaN materials is essential to the advancement of GaN technology. In this thesis, the effect of temperature on the optical and electrical properties of p-type GaN is investigated. The GaN samples used in this work were grown by various methods and studied by Kelvin probe and photoluminescence (PL) techniques. Specifically, the surface photovoltage (SPV) behavior and PL data were analyzed at different temperatures and illumination intensities. Using the SPV results, we show that p-type GaN exhibits n-type conductivity at low temperatures (80 K). If the sample is heated beyond a characteristic temperature, TC, the conductivity reverts to p-type. This temperature of conversion can be tuned by varying the illumination intensity. We explain this conductivity conversion using a simple, one-acceptor phenomenological model. Temperature-dependent PL measurements taken on Mg-doped p-type GaN layers show abrupt and tunable thermal quenching of the PL intensity. This effect is explained by a more complex model but with the same assertions, that the system must undergo a change in conductivity at low temperatures and under UV illumination. It is necessary to understand the observed behaviors, since the implications of such could have an effect on the performance of devices containing p-type GaN materials

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen--vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.Comment: Review paper, 36 page