Density functional theory is used to study the physico-chemical effects of two metallic thin films separated by distances in a range of 4-10 amperes. In this condition, the electrons from the metallic thin film surfaces tunnel through the energy barrier existing between the separated thin films, creating an electronic distribution in the gap between films. The characteristics and features of this electronic distribution, such as energy, momentum, and number of electrons, can be traced by quantum mechanical analyses. These same features can be tuned by varying metallic thin film properties like thickness, separation between films, and film chemical nature. The possibility to tune the physical properties of the electrons located in the gap between thin films makes the studied systems promising for applications that range from catalysis to nano-electronics. Molecular oxygen, water, and ethylene were located in the gap between thin films in order to study the physical and chemical effects of having those molecules in the gap between thin films. It was observed that the electron structure in the gap modifies the geometric and electronic structure of those molecules placed in the gap. In the case of molecular oxygen, it was found that the dissociation energy can be tuned by changing the separation between thin films and changing the chemical nature of the surface and overlayer of the thin film. For water, it was found that by tuning the chemical nature of the surface and sub-surface of both metallic thin films, molecular water dissociation can occur. When ethylene was located in the gap between Ti/Pt thin films, the molecule converts in an anion radical adopting the geometry and structure of the activated monomer necessary to initiate chain polymerization. Regarding magnetism, it was found that by the surface interaction between Ti/Pt and Pt thin films, the magnetic moment of the system decreases as the separation between thin films decreases. The phenomenon was explained by changes observed in the number of electronic states at the Fermi level and in the exchange splitting as a function of separation between films. Finally, a system that resembles a p-n junction was proposed and analyzed. The system is a junction of two metallic thin films with different electronic density in the gap between surfaces. These junctions can be the building blocks for many electronic devices.

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Thesis (M.S.); Submitted to Iowa State Univ., Ames, IA (US); 19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2498" Seymen Murat Aygun. US Department of Energy 12/19/2004. Report is also available in paper and microfiche from NTIS.

Germanium (Ge) is the most compatible semiconductor material with silicon-based complementary metal-oxide semiconductor technology, which has higher electron and hole mobility than Si, leading to enhanced device performance. In addition, semiconductor nanowires (NWs) have attracted significant attention as promising candidates for next-generation nanoscale devices. Due to their unique geometry and physical properties, NWs show excellent optical and electrical properties such as quantum size effects, enhanced light absorption, and high biological and chemical sensitivity. Furthermore, high response to light irradiation is one of the most significant properties of semiconductor NWs, which makes them excellent candidates for photodetectors. Hence, Ge NWs are promising high-mobility nanostructures for optoelectronic devices. Despite constant improvement in the performance of single NW-based devices, determining their electrical properties remains challenging. Here, a symmetric six-contact Hall bar configuration is developed for top-down fabricated highly doped Ge NWs with different widths down to 30 nm, which simultaneously facilitates Hall effect and four-probe resistance measurements. Furthermore, accurate control of doping and fabrication of metal contacts on n-type doped Ge NWs with low resistance and linear characteristics remain significant challenges in Ge-based devices. Therefore, a combined approach is reported to fabricate Ohmic contacts on n-type doped Ge NWs using ion implantation and rear-side flash lamp annealing. This approach allows the fabrication of axial p–n junctions along the single NWs with different widths. The fabricated devices demonstrated rectifying characteristics in dark conditions. The photoresponse of the axial p–n junction photodetectors was investigated under three different illumination wavelengths of 637 nm, 785 nm, and 1550 nm. Moreover, the fabricated axial p–n junction photodetector demonstrated a high-frequency response up to 1 MHz at zero bias.

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-13T12:17:27Z No. of bitstreams: 1 arquivototal.pdf: 26917769 bytes, checksum: 9ff17103475ce4130305b157369d8448 (MD5) / Made available in DSpace on 2017-09-13T12:17:27Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 26917769 bytes, checksum: 9ff17103475ce4130305b157369d8448 (MD5) Previous issue date: 2015-03-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In the present work, we use first-principles calculations based on density functional theory, as implemented in the SIESTA code, to investigate the changes in the structural and electronic properties of the carbon, BN, and hybrid BxCyNz nanotubes produced by one or two of the following mechanisms: doping with carbon atoms, the application of external electric fields, by flattening of the cross section, the encapsulation of a carbon nanowire or the adsorption of hydrogen atoms (hydrogenation). We start with the study of double-walled boron nitride nanotubes (DWBNNTs), zig-zag and armchair, doped with carbon atoms, with chiral vectors (8,0)@(16,0) and (5,5)@(10,10), respectively. Two types of doping were considered: one C atom substituting a B atom on the inner wall (IW) and one C atom substituting a N atom on the outer wall (OW), which we call of CB[IW]@CN[OW], and the opposite situation results in CN[IW]@CB[OW]. In this sense, we generate a (type-p semiconductor)@(type-n semiconductor) and a (type-n semiconductor)@(type-p semiconductor), where the resulting DWBNNTs can be thought of as p-n junctions. At the same time, we apply an external electric field, with magnitude of 0,3 V/Å, in different directions, namely, perpendicular (Ey), parallel (Ex), and antiparallel (E􀀀x) to the line formed by the dopants. Thus, depending on the direction of the applied field, we observe an increase or decrease in the band gap energy between the defect levels (Eig), and such cases are related to the reverse and direct polarization of the p-n junction, respectively. Afterwards, we study the insertion of a carbon nanowire (CNW) inside a (10.0) zigzag carbon nanotube and inside a (10.0) zig-zag BN nanotube. Such systems were called CNW@SWCNT and CNW@SWBNNT, respectively. We produce the flattening of the nanotubes and verify the behavior of the atomic structure of the nanowire as the flattening of the nanotube increases. From the obtained results, it was possible to conclude that, for both CNW@SWCNT and CNW@SWBNNT, there is a critical distance dc (distance between the parallel planes of the flattened nanotubes (d)), with the value of 3.60 Å, so that we can summarize our findings as follows: in the case d > dc, the carbon nanowire does not undergo any deformation; and in the reverse case (d < dc), the carbon nanowire binds to the wall of the nanotube and undergoes deformations. Regarding the electronic properties, we verify that the encapsulation of the CNW inside the SWCNT and SWBNNT, produces a significant reduction of the band gap energy (Eg) of such systems. Moreover, we observe ABSTRACT viii the creation of Dirac points for some flattening ratios of the nanotubes. Finally, we carry out a study on the adsorption of hydrogen atoms (hydrogenation) on the surface of double-walled boron nitride nanotubes (DWBNNTs) and hybrid nanotubes of boron nitride and carbon (DW(BN)xCyNTs). Due to the fact that the nanotubes have two walls, we consider the following cases: (i) coverages of 2H, 4H, 8H, 12H, and 16H on the inner wall, (ii) coverages of 2H, 4H, 8H, 16H, and 32H on the outer wall, and (iii) coverages of 2H, 4H, 8H, 16H, and 32H on both walls. Curiously, we find that for all hydrogen coverages considered, a strong deformation occurs in the hydrogen regions, causing the cross section of the nanotubes take different polygonal shapes: ellipsoidal, rectangular, hexagonal or octahedral. For coverages of 16H and 32H only on the outer wall, we observe that some hydrogens desorbed from the wall forming isolated H2 molecules without preferential orientation. We verify that, in some cases, the bond angles between the B, N and H or C and H atoms exhibit characteristics of the sp3 hybridization. Regarding the structural stability, we verify that the adsorption of H atoms in DWBNCNTs is more favorable than in DWBNNTs. Moreover, we conclude that is possible to control the band gap energy of the nanotubes through the hydrogen coverage. / No presente trabalho, usamos cálculos de primeiros princípios baseados na Teoria do Funcional da Densidade, como implementado no código SIESTA, para investigarmos as alterações nas propriedades estruturais e eletrônicas de nanotubos de carbono, de BN e híbridos BxCyNz, produzidas por um ou dois dos seguintes mecanismos: dopagem com átomos de carbono, aplicação de campos elétricos externos, pelo achatamento da secção transversal, encapsulamento de um nanofio de carbono ou pela adsorção de átomos de hidrogênio (hidrogenação). Iniciamos com o estudo de nanotubos de nitreto de boro de parede dupla (DWBNNTs), zig-zag e armchair, dopados com átomos de carbono, com vetores quirais (8,0)@(16,0) e (5,5)@(10,10), respectivamente. Duas situações de dopagem foram consideradas: um átomo de C substituindo um átomo de B na parede interna (IW) e um átomo de C substituindo um átomo de N na parede externa (OW) a qual chamamos de CB[IW]@CN[OW], e a situação oposta resulta em CN[IW]@CB[OW]. Neste sentido, construímos um (semicondutor do tipo-p)@(semicondutor do tipo-n) e um (semicondutor do tipo-n)@(semicondutor do tipo-p) onde os DWBNNTs resultantes podem ser pensados como junções p-n. Paralelamente, aplicamos um campo elétrico externo, com magnitude de 0,3 V/Å, em diferentes direções, a saber, perpendicular (Ey), paralelo (Ex) e anti-paralelo (E􀀀x) a linha formada pelos dopantes. Assim, dependendo da direção do campo aplicado, observamos um aumento ou diminuição do gap de energia entre os níveis de defeitos (Eig) e tais casos estão relacionados a polarização reversa e direta da junção p-n, respectivamente. Em seguida, estudamos a inserção de um nanofio de carbono (CNW) no interior de um nanotubo de carbono e de BN, ambos com vetor quiral (10.0). Tais sistemas foram chamados de CNW@SWCNT e CNW@SWBNNT, respectivamente. Nós produzimos o achatamento dos nanotubos e verificamos o comportamento da estrutura atômica do nanofio a medida que o achatamento do nanotubo aumenta. A partir dos resultados obtidos, foi possível concluir que para ambos os CNW@SWCNT e CNW@SWBNNT, existe uma distancia crítica dc (distância entre os planos paralelos dos nanotubos achatados (d)), com um valor de 3.60 Å, de tal forma que nós podemos resumir as nossas descobertas como: no caso de d > dc, o nanofio de carbono não sofre nenhuma deformação; e no caso reverso (d < dc), o nanofio de carbono liga-se a parede do nanotubo e sofre deformações. Em relação as propriedades eletrônicas, verificamos que o encapsulamento do CNW nos SWCNT e SWBNNT, produz uma significativa redução do gap de energia (Eg) de tais sisteRESUMO vi mas. Além disso, observamos a formação de pontos de Dirac para algumas taxas de achatamento dos nanotubos. Por último, nós realizamos um estudo da adsorção de átomos de hidrogênio (hidrogenação) sobre a superfície de um nanotubo de parede dupla de nitreto de boro (DWBNNTs) e um nanotubo híbrido de nitreto de boro e carbono (DW(BN)xCyNTs). Devido ao fato dos nanotubos possuírem duas paredes, consideramos os seguintes casos: (i) coberturas de 2H, 4H, 8H, 12H e 16H na parede interna, (ii) coberturas de 2H, 4H, 8H, 16H e 32H na parede externa e (iii) coberturas de 2H, 4H, 8H, 16H e 32H em ambas as paredes. Curiosamente, verificamos que em todas as coberturas de hidrogênio consideradas, uma forte deformação ocorre nos locais de hidrogênio, fazendo a secção transversal dos nanotubos se transformar em diferentes formas poligonais: elipsoidal, retangular, hexagonal ou octaedral. Para coberturas de 16H e 32H apenas na parede externa, observamos que alguns hidrogênios se dessorveram da parede formando moléculas de H2 isoladas sem orientação preferencial. Verificamos que em alguns casos, os ângulos de ligação entre os átomos de B, N e H ou C e H exibem características da hibridação sp3. Com relação a estabilidade estrutural, verificamos que a adsorção de átomos de H em DWBNCNTs é mais favorável do que em DWBNNTs. Ademais, concluímos que é possível controlar o gap de energia dos nanotubos através da cobertura de hidrogênio.