Tek Duvarlı Karbon Nanotüplerin Elektronik Yapılarına Göre Ayrılması Ve Karakterizasyonu

Abstract

Karbon ailesinin tek boyutlu allotropu olan karbon nanotüpler, mükemmel mekanik, termal, elektriksel ve optik özellikleri nedeniyle keşfedilmesinden bu yana pek çok araştırmaya konu olmaktadır. Sentez yöntemlerine bağlı olarak seri üretimi gerçekleştirilebilen nanotüplerin, kullanım alanına yönelik tek ya da çok duvarlı yapıları elde edilmektedir. Ancak, tek duvarlı karbon nanotüpler (TDKNT), çaplarına ve kiralitelerine göre değişim gösteren metalik karakterin yanı sıra yarı iletken özelliğe sahip olması ile bilim dünyasında oldukça ilgi çekmektedir. TDKNT'lerin çeşitli alanlarda uygulamasının yüksek verimlerde gerçekleştirilmesi, elektronik yapılarına göre ayrılmasında büyük önem taşımaktadır. Bu amaçla, ayırma işlemlerinde, TDKNT'lerin kristalografik yapısı etkin rol oynamakla birlikte belirli parametreler göz önünde bulundurularak değişik fiziksel ve kimyasal yöntemler kullanılmaktadır. Bu işlemler, nanotüpler ile ayırma işlemlerinde uygulanmak üzere sentezlenen bileşiklerin moleküler etkileşimini arttırmaya yönelik olup nanotüpün kiralitesi ve çapına bağlı olarak farklı sonuçların elde edilmesine yol açmaktadır. Böylelikle, mevcut yöntemler ile istenilen çapta ve kiralitede, yüksek saflıkta yarı iletken TDKNT üretimine ilişkin standart bir metodoloji günümüzde henüz geliştirilememiştir. Ayrıca, literatürde, ayrımın gerçekleşmesine dair mekanizmanın açıklandığı sınırlı sayıda çalışma bulunmaktadır. Bu çalışmada, jel kromatografi yöntemi kullanılarak nanotüp-sabit faz ve nanotüp-yüzey aktif madde arasındaki moleküler etkileşim araştırılmıştır. Jel kromatografi yönteminde TDKNT'lerin elektronik yapılarına göre ayrımı, anyonik yüzey aktif madde varlığında dispersiyon işlemi gerçekleştirilmesinin ardından kolon yardımıyla sabit faz olarak kullanılan sefakril 200 jel ortamında, hareketli fazı ikili sistemin oluşturduğu anyonik yüzey aktif maddeler ile farklı derişimlerde hazırlanarak gerçekleştirilmiştir. Ayrıca, literatürden farklı olarak kromatografi yönteminde etkin bir ayrılmanın sağlanması için önemli olan dispersiyona, iyonik sıvının (N,N,N-tribütil amonyum 9-oktadekenoat) etkisi incelenmiştir. Metalik ve yarı iletken özelliklerine göre ayrımı sağlanan TDKNT'lerin karakterizasyon işlemleri UV-vis-NIR ve Raman spektroskopi yöntemleriyle gerçekleştirilmiştir. Optik absorpsiyon spektroskopisi yöntemiyle metalik ve yarı iletken TDKNT'lerin analizi 400-1350 nm dalga boyları arasında yapılmış olup metalikçe zengin TDKNT'lerin 400-620 nm (M11), yarı iletkence zengin TDKNT'lerin 850-1350 nm (S11) dalga boyunda absorpsiyon pikleri gözlenmiştir. Raman spektrumları ise literatür ile paralel olarak; nanotüpe özgü olan radyal soluklanma modu (RBM) bandı piki 100-500 cm−1 aralığında, sivri ve şiddetli G bandı piki 1550-1595 cm−1 aralığında ve yapıdaki kusurları gösteren D bandı piki 1250-1450 cm−1 aralığında oldukça zayıf ve küçük şekilde gözlenmiş olup bu durum ayırma işlemleri sonucunda yapıda önemli ölçüde bir hasar meydana gelmediğinin göstergesidir. Kalitatif analizin temel alındığı çalışmada, ayırma verimi, ticari olarak kullanılan malzemenin analiz sonuçları karşılaştırılarak değerlendirilmiştir. Yapılan çalışmada ticari TDKNT ile karşılaştırıldığında, (çaptan kaynaklanan) farklı kiralitede ve yarı iletkence zengin TDKNT nanotüplerin elde edildiği gözlemlenmiştir.

Nanotechnological researches increase progressively directed towards especially carbon nanotubes due to their unique and outstanding properties. Although, chemical composition and atomic bonding configuration of nanotubes are not complicated to analyze, they display extraordinary features. Therefore, structures, sythesis, growth mechanisms, and factors that affecting properties of carbon nanotubes have attracted enormous attention for widespread applications. Carbon nanotubes are composed of rolling up graphene sheet into a cylinder with the hexagonal rings. They can divided by two structure in terms of tube layer; single-walled carbon nanotube (SWNT) which contain one layer and multi-walled carbon nanotube (MWNT) which contain several layers. SWNTs diameter range is approximately 0.4-3 nm, while MWNTs is about 2-50 nm related to number of layers in the nanotube structure. SWNT structure is stated by diameter, lenght and chiral angle depending on helicity of the graphene sheet. Because of the fact that SWNT include long periodical unit (high lenght), it might be analyzed as one dimensional nanostructure. Optical and elecetronic properties of SWNTs are unique because of the one-dimensional (1D) structure in phonon states appearing van Hove singularities (vHSs) in the density of states (DOS). SWNTs are attracted for electronic applications because of their extraordinary charge carrier mobilities and solution processiblity. SWNT synthesis methods include the arc discharge method, laser ablation and chemical vapor deposition with diverse diameter and chirality range. Synthesis methods developed so far are incapable of producing SWNTs of selective enrichment structures at significant scale and therefore, separation of synthetic mixtures of SWNTs is both scientifically interesting and technologically important. On the basis of their electronic structures, SWNTs can be classified into two categories: metallic (m-SWNTs) and semiconducting SWNTs (s-SWNTs). Because pure s-SWNTs are required for a range of electronic applications, the ability to obtain pure s-SWNTs from as-synthesized SWNT mixtures is very important. Diameter and tube's chirality (n,m) of SWNTs play a tremendous role of the electronic structure and optical properties. Although the structure of the SWNTs is determined related to the (n, m) chirality, it doesn't include any information about helicity of the nanotube that categorized the symmetry as chiral or achiral. Chiral species are detected with two enantiomers of a specific (n, m)-SWNTs while achiral species are described by angles of 00 (zigzag nanotubes, m=0) and 300 (armchair, n = m). SWNTs (as synthesized) sample are mainly fabricated as a mixture containing two third of semiconducting and one third of metallic species with wide range of chiralities. Therefore, separation of SWNTs is essential for potential application of SWNTs. Even though several types of SWNTs separation exist, mechanism of these procedures is unclear and to improve standart methodology is fundamental and challenging problem. The dispersion of SWNTs has been a key research topic over the past decade. Methods for separating s-SWNTs include the sorting of SWNTs in solution, the removal of metallic SWNTs after growth, interactions via surface functional groups and chemical reactions with SWNTs. The solution-based sorting of s-SWNTs via noncovalent functionalization has been shown to be an excellent method for selecting pure s-SWNTs without altering their electrical properties. These solution- based sorting methods include the wrapping of SWNTs with DNA molecules, density gradient ultracentrifugation, gel chromatography, partition separation and the wrapping of SWNTs with a conjugated polymer that several methods are even capable of sorting single-chirality SWNTs. Among these methods, gel chromatography is one of the simplest, cheap and highly scalable method for the large-scale sorting of SWNTs. In this study, separation of SWNTs according to electronic structure was achieved via gel chromatography method. First of all, HiPco SWNTs were dispersed in sodium dodecyl sulfate (SDS) using with dispersion process (ultrasonication and ultracentrifugation). Besides, ionic liquid (N,N,N-tributyl ammonium 9-octadecenoate) consisting of ammonium group (cationic) was used for the first time dispersion of SWNTs by applying same strategy. Although, dispersion achieved by ionic liquid unfortunately, gel chromatography method was not used that of solution mixture because of the precipitation after ultracentrifugation process. As a second step related to the separation strategy, dispersions of nanotubes in the surfactant SDS were passed through a gel matrix which is usually composed of crosslinked allyl dextran gel beads (sephacryl 200). In this case, metallic species passed through the gel and were obtained in the initial eluate, while semiconducting species were adsorbed to the stationary phase and collected by SDS and sodium cholate (SC) with different concentrations. Several runs were applied until no SWNTs weren't adsorbed to the gel. After that, the remaining unadsorbed SWNTs in the bottom phase were passed through sepharose 2B gel which was used as secondary gel in the column with the same eluent system. The mechanism behind the separation is believed to be related to conformational differences between SDS adsorbed on metallic and semiconducting species, rather than any size exclusion effects due to selective aggregation or dispersion of either nanotube type. Many factors may play a role in the separation mediated by SDS, such as interactions between SWCNTs and SDS or interactions between SWNTs and hydrogels in SDS solution. The difference between m- and s-SWCNTs under such surfactant conformations was suggested to affect the tubes interaction with the hydrogels and thus result in separation. Furthermore, the interaction of surfactants with hydrogels should be possible. It was found that surfactant concentration affect interactions between hydrogel and nanotubes. Increasing SDS concentration provides s-SWNT desorption from the gel accordingly. Also, sodium cholate (SC) as a second surfactant provides desorption of remaining s- SWNTs from the gel. The separation achieved was verified by UV-vis- NIR and Raman spectroscopy. Optical absorption data were recorded from 400 to 1350 nm with an UV − vis − NIR spectrophotometer using a quartz cell. Pristine HiPco SWNTs were dispersed in 1 wt % aqueous SDS solution. For the HiPco SWNTs, the absorption peaks at 850 – 1350 nm were derived from the first (S11 ), optical transitions of the s-SWNTs.. The absorbance peak at 400 − 620 nm represented the first optical transition of metallic m-SWNTs (M11). According to Raman spectrum, the radial breathing mode (RBM) peak was observed at 100 – 500 cm−1, the D-band peak which indicates impurities in SWCNT solution was observed at 1250 − 1450 cm−1 with very low intensity, the G-band peak was observed at 1550 − 1595 cm−1. In this study, qualitative analysis were conducted for separation of SWNTs, however, separation efficiency evaluated comparing commercial HiPco SWNTs data. As a result, semiconducting enriched SWNTs were obtained with diverse chiral species because of the diameter range differences.