Computer simulation of electromigration induced void grain boundary interactions and the prediction of cathode failure times in bamboo structures

Ph.D. - Doctoral Progra

Few layer graphene synthesis via SiC decomposition at low temperature and low vacuum

Based on the large-scale availability and good electrical properties, the epitaxial graphene (EG) on SiC exhibits a big potential for future electronic devices. However, it is still necessary to work continuously on lowering the formation temperature and vacuum values of EG while improving the quality and increasing the lateral size to fabricate high-performance electronic devices at reduced processing costs. In this study, we investigated the effect of the presence of Mo plate and hydrogen atmosphere as well as the vacuum annealing durations on SiC decomposition. Our studies showed that the graphene layers can be produced at lower annealing temperatures (1200 degrees C) and vacuum values (10(-4) Torr) in the presence of Mo plate and hydrogen. For high quality continuous graphene formation, Mo plate should be in contact with SiC. If there is a gap between Mo and SiC, non-wetting oxide droplets on few layer graphene (FLG) are recorded. Moreover, it is found that the morphology of these islands can be controlled by changing the annealing time and atmosphere conditions, and applying external disturbances such as vibration

İnce film metalik iletişim bağlarında oluşan elektrogöç güdümündeki iç makro boşlukların ve yüzey tane oluklarının hidroastatik ve çift akslı gerilimler altındaki dinamiklerinin matematik modellemesi ve bilgisayar benzetimi

TÜBİTAK MAG Proje31.08.200

Stranski-Krastanow islanding initiated on the stochastic rough surfaces of the epitaxially strained thin films

Quantum dots (QD) have discrete energy spectrum, which can be adjusted over a wide range by tuning composition, density, size, lattice strain, and morphology. These features make quantum dots attractive for the design and fabrication of novel electronic, magnetic and photonic devices and other functional materials used in cutting-edge applications. The formation of QD on epitaxially strained thin film surfaces, known as Stranski-Krastanow (SK) islands, has attracted great attention due to their unique electronic properties. Here, we present a systematic dynamical simulation study for the spontaneous evolution of the SK islands on the stochastically rough surfaces (nucleationless growth). During the development of SK islands through the mass accumulation at randomly selected regions of the film via surface drift-diffusion (induced by the capillary and mismatch stresses) with and/or without growth, one also observes the formation of an extremely thin wetting layer having a thickness of a few Angstroms. Above a certain threshold level of the mismatch strain and/or the size of the patch, the formation of multiple islands separated by shallow wetting layers is also observed as metastable states such as doublets even multiplets. These islands are converted into a distinct SK islands after long annealing times by coalescence through the long range surface diffusion. Extensive computer simulation studies demonstrated that after an initial transient regime, there is a strong quadratic relationship between the height of the SK singlet and the intensity of the lattice mismatch strain (in a wide range of stresses up to 8.5GPa for germanium thin crystalline films), with the exception at those critical points where the morphological (shape change with necking) transition takes place. (C) 2014 AIP Publishing LLC

Mesoscopic nonequilibrium thermodynamics treatment of the grain boundary thermal grooving induced by the anisotropic surface drift diffusion

A systematic study based on the self-consistent dynamical simulations is presented for the grain boundary thermal grooving problem by strictly following the irreversible thermodynamic theory of surfaces and interfaces with singularities [T. O. Ogurtani, J. Chem. Phys. 124, 144706 (2006)]. This approach furnishes us to have auto-control on the otherwise free-motion of the grain boundary triple junction without presuming any equilibrium dihedral (wetting) angles at the edges. The effects of physicochemical properties and the anisotropic surface diffusivity on the transient grooving behavior, which takes place at the early stage of the scenario, were considered. We analyzed the experimental thermal grooving data reported for tungsten in the literature, and compared them with the carried simulation results. This investigation showed that the observed changes in the dihedral angles are strictly connected to the transient behavior of the simulated global system, and manifest themselves at the early stage of the thermal grooving phenomenon

Grafen – Bakır Lamine Yapılı Isı Dağıtıcı Plaka Geliştirilmesi

Günümüz mikroislemci teknolojilerinde, islemci bölgesinde olusan ısıyı, bu alandan uzaklastırmak için bakır gibi yüksek termal iletkenlige sahip ısı dagıtıcı plakalar kullanılmaktadır; ancak bakırın, artan akım kaynaklı ısı yükleri karsısında giderek yetersiz kalması, bu kullanım alanında alternatif malzeme arayısını zorunlu kılmıstır. Bu proje kapsamında, Aselsan?da aktif olarak elektronik kartlarda kullanılmakta olan bakır ısı dagıtıcının yerine; yanal yöndeki ısı iletimi arttırılarak ısı dagıtıcının tüm alanının efektif bir sekilde kullanımını saglayacak, yenilikçi bir malzeme gelistirilmesi amaçlanmıstır. Bu baglamda, Grafen-Cu lamine yapı tasarımı önerilmistir. Bunun için öncelikle genis kapsamda bakır yüzeyinde grafen olusumu üzerine çalısılmıs, sentez ve karakterizasyon çalısmaları ile grafen büyümesinde baskın deneysel parametrelerin etkileri sistematik bir sekilde incelenmis ve kimyasal buhardan çöktürme (KBÇ) yöntemi ile grafenin iki boyutlu çekirdeklenme ve büyüme mekanizması belirlenerek, bakır folyo yüzeyinde olabildigince sürekli ve kaliteli grafen sentezi için parametreler basarıyla optimize edilmistir. Öte yandan, hesaplamalı yöntemler ve deneysel termal ölçümler sonucunda Grafen-Cu lamine yapısında grafenin sistemde Cu katmana göre çok ince kalmasından dolayı, termal iletimde beklenen artısı saglayamadıgı görülmüstür. Böylece çalısmalardan edinilen bilgi ısıgında Grafen-Cu Köpük ve Cu-Grafen Sandviç olmak üzere iki yeni yapı tasarımı önerilmis, bunlardan Grafen-Cu Köpük sentezi, karakterizasyonu ayrıntılı sekilde çalısılarak, termal ölçümleri gerçeklestirilmis ve bu yapıda grafenin termal yayınım performansının tüm sıcaklık aralıgında (Oda sıcaklıgından 400 oC?ye kadar) arttırdıgı gösterilmistir. Grafenin termal özelliklere bu pozitif etkisi, grafenin yüksek termal yayınımının yanı sıra, bakırı oksitlenmeden koruması ile de açıklanmıstır.In today's technology, heat spreaders, made from materials with high thermal conductivity values (e.g. copper), play important role in removing the heat generated within the microprocessor (CPU). However, these materials quickly became inefficient to dissipate the ever increased heat loads, hence novel alternative materials, with higher dissipation efficiency are required. In this context, in the proposed project, our aim is to develop a novel heat spreader made of graphene-copper laminate structure as better alternative to copper, which is the most common heat spreader material used in Aselsan. Here, we aim to increase the lateral heat conduction in a way to ensure the effective use of the entire area of the heat spreader. More rapid spread of heat in the lateral direction of the graphene-copper foil laminate system will lead to increased removal of the total heat. For this, firstly the formation of graphene on Cu foil is studied. The effects of the major processing parameters and the mechanisms were investigated. Hence the graphene synthesis on Cu foil was optimized for the futher Graphene-Cu laminate structures. However, the computational methods and experimental thermal measurements showed that the graphene in the graphene-Cu laminate structure was too thin compared to the Cu layer in the system to achieve the expected increase in thermal conduction. Thus, in the light of the results obtained from the studies, two new structural designs, Grafen-Cu Foam and Cu- Grafen Sandwich, were proposed. The Graphene-Cu Foam synthesis and characterization were studied in detail and thermal measurements were carried out. The thermal diffusivity performance of the structure with graphene has been shown to increase over the entire temperature range (from room temperature to about 400 oC). This positive effect of graphene on thermal properties is explained by the high thermal diffusivity of graphene as well as its protection of copper from oxidation. Keywords: Graphene, copper, laminate structure, heat spreader, graphene-copper foam, chemical vapor depositio

Graphene/Copper Heterostructures for Thermal Management

With the technological developments in the microelectronic systems used in military computers, the number of circuit elements per unit area increases enabling the production of faster and more efficient processors. To be able do this, these circuit elements are required to withstand higher current densities and thus higher temperatures are generated by Joule heating. Overheating (in general non-uniformly) at some specific areas in chips, adversely affects the performance and reliability of electronic devices. Therefore, it is critical to control temperature distribution within the chip and the efficient heat management is one of the most important issues for today’s high power electronic devices and thus, every improvement in the area is very valuable. In this context, to increase the lateral heat conduction, the graphene-copper heterostructures (graphene-copper laminate structures for heat spreaders and graphene-copper porous structures for heat sinks/exchangers) are studied both experimentally and through computational studies. For the experimental studies, first graphene is synthesized on Cu via CVD. The thermal diffusivity measurements, which were performed through the laser flash method, show that the presence of graphene did not make a contribution to the thermal properties in graphene-copper laminate system. These results were also confirmed by the computational studies which showed that to see an increase in the thermal conductivity, the ratio of graphene/copper should be higher than 1/20. Within the scope of these findings, 3D graphene-Cu porous heterostructures are studied to increase the graphene’s contribution to the thermal diffusivity. 3D graphene-Cu porous heterostructures showed an increase in the thermal diffusivity by 10% at the room temperature and 30% at 400 °C. Graphene’s positive effect on the thermal properties is attributed to its high thermal conductivity and the protection of Cu structure against the oxidation at higher temperatures. Our studies show that the graphene-copper porous structures developed in this study can be a good lightweight candidate for a heat sink/exchanger with corrosion resistant and high thermal conductivity

Engineered Chimeric Peptides as Antimicrobial Surface Coating Agents toward Infection-Free Implants

Prevention of bacterial colonization and consequent biofilm formation remains a major challenge in implantable medical devices. Implant-associated infections are not only a major cause of implant failures but also their conventional treatment with antibiotics brings further complications due to the escalation in multidrug resistance to a variety of bacterial species. Owing to their unique properties, antimicrobial peptides (AMPs) have gained significant attention as effective agents to combat colonization of microorganisms. These peptides have been shown to exhibit a wide spectrum of activities with specificity to a target cell while having a low tendency for developing bacterial resistance. Engineering biomaterial surfaces that feature AMP properties, therefore, offer a promising approach to prevent implant infections. Here, we engineered a chimeric peptide with bifunctionality that both forms a robust solid-surface coating while presenting antimicrobial property. The individual domains of the chimeric peptides were evaluated for their solid-binding kinetics to titanium substrate as well as for their antimicrobial properties in solution. The antimicrobial efficacy of the chimeric peptide on the implant material was evaluated in vitro against infection by a variety of bacteria, including Streptococcus mutans, Staphylococcus. epidermidis, and Escherichia coli, which are commonly found in oral and orthopedic implant related surgeries. Our results demonstrate significant improvement in reducing bacterial colonization onto titanium surfaces below the detectable limit. Engineered chimeric peptides with freely displayed antimicrobial domains could be a potential solution for developing infection-free surfaces by engineering implant interfaces with highly reduced bacterial colonization property

Peptides to bridge biological-platinum materials interface

Peptides with inorganic materials recognition already started to impact a wide range of surface- related technologies ranging from biomonitoring to biomedical areas. Combinatorial biology- based libraries are the initial step in tempting the directed evolution of peptides with specifi c interactions towards technologically relevant materials. Here, a case study is provided to demonstrate the specifi c peptide binding and the amino acids residues that play an important role for platinum surface affi nity by combining computational as well as genetic engineering tools. Using a phage display technique, septapeptides were identifi ed exhibiting affi nity to noble metal platinum, and the amino acid distributions in the identifi ed peptides were analyzed. The analysis of the peptide sequences showed that strong Pt- binding peptides contain positively charged, hydrophilic, and polar residues, and especially enriched in threonine, serine, and glutamine. Under competitive surface- binding conditions, strong Pt- binding peptide motif displayed on phage resulted in high specifi city to Pt regions on a Pt- macropatterned glass. Conformational analysis of the strong binder indicates that threonine and serine as well as glutamine are in close contact with the surfaces forming a tripod molecular architecture. The alanine substitution mutagenesis applied at the genomic level to the peptide displayed on the phage revealed threonine and serine substitutions as the critical ones. Understanding the residue- based interactions of the peptide sequences can be utilized to tune the affi nity and the specifi city of the peptides with the inorganic surfaces, toward making them indispensable molecular tools to control the molecular interactions of biological macromolecules with the material surfaces