34 research outputs found
The effect of experimental unilateral cryptorchidism on the contralateral exocrine testicular function in arabbit model
No full textThe effect of experimental unilateral cryptorchidism on the contralateral exocrine testicular function in a rabbit model. Dimitriadis Fotios MD FEBU It is well known that subfertility is frequently observed in cases of cryptorchidism. The degenerative changes which occur in the cryptorchid testis have been noted by many investigators and are believed to be a cause of male infertility in bilateral cryptorchidism. However, the matter of fertility in patients with unilateral cryptorchidism continues to be argued, although several studies have shown a decrease in fertility in patients with unilateral undescended testis. The most debated issues include: a) the existence or not of the decreased fertility in these patients as a result of the bilateral effect of the unilateral undescended testis, b) the role of the undescended testis in the contralateral testicular damage or the presents of other reasons such as underlying congenital defect, c) the rate of the germ cells and in particular spermatogenic sperm cells (that is spermatogonia) which survive the cryptorchid state, d) the extend of the contralateral testicular damage, and e) the pathogenesis of the decreased fertility in these patients. In the present study we used rabbit model of intra-abdominal unilateral cryptorchidism to compare the fertility, hormonal profile, and histological changes of both the undescended and descended testes by means of light microscopy. Unilateral cryptorchidism was induced in twenty male 5 month old New Zealand white rabbits (Group I) by returning the right testis, together with the ipsilateral epididymis and spermatic cord, to the abdominal cavity through an inguinal incision. The inguinal ring was closed with a suture. Another twenty male age-matched New Zealand white rabbits (Group II) underwent sham operation and served as control group. All operations were performed by an experienced surgeon and anesthesia was induced by intramuscular administration of ketamine 15mg/kg body weight as anesthetic followed by subcutaneous administration of xylazine 15mg/kg body weight as pain killer and muscle relaxant as well as subcutaneous administration of heparin 120 U/Kg body weight as anticoagulant. One hundred and nineteen days after the experiment, the fertility of both groups was assessed by mating 10 rabbits from each group selected randomly with mature females of proven fertility (two successful parturitions in the past). The next day (120 days after the experiment) peripheral blood sample was aspirated from an ear vein for the assessment of basal testosterone level. After one day (121 days after the experiment) hCG (5000U/animal) was administered intramuscularly in all male animals and three hours later peripheral blood sample was aspirated to assess the response of testosterone. The next day (122 days after the experiment) all the male animals were killed and all the testes were extracted, weighed, and processed for histological evaluation. The last day of the experiment (144th day) all the female animals were killed and the number of embryos inside the uterus was registered. Bilateral testicular weight was significantly smaller in group I than in group II. In addition mean seminiferous tubule diameter, mean seminiferous tubule area, mean number of round spermatids per seminiferous tubule, and mean number of primary spermatocytes/seminiferous tubule was significantly smaller bilaterally in group I than in group II. Furthermore basal testosterone levels and testosterone responses to hCG stimulation were significantly smaller in group I than in group II. Moreover, the proportion of fertile rabbits to the total number of rabbits was significantly smaller in group I than in group II. These results suggest that intra-abdominal cryptorchid testes are significantly impaired, and that such impairment might be caused by exposure of the testis to a higher temperature. The more severely impaired undescended testes may thus induce the degeneration of the exocrine function of the contralateral scrotal testis and thereby cause subfertility in the intra-abdominal unilateral cryptorchid rabbit model. Moreover, the results of the current study provide some evidence of the detrimental effects of unilateral cryptorchidism on testicular endocrine function of the contralateral testis.Ο σκοπός της παρούσας πειραματικής διατριβής ήταν η μελέτη της εξωκρινούς μοίρας του όρχη σε κουνέλια με κρυψορχία του δεξιού όρχη. Χρησιμοποιήθηκαν σαράντα αρσενικά κουνέλια (New Zealand White Rabbits) ηλικίας πέντε μηνών τα οποία χωρίστηκαν σε δύο ομάδες των είκοσι ζώων. Τα κουνέλια της ομάδας Ι υποβλήθηκαν σε επέμβαση πρόκλησης δεξιάς κρυψορχίας με επιμήκη τομή στο ύψος του έσω δακτυλίου του δεξιού βουβωνικού καναλιού. Τα κουνέλια της ομάδας ΙΙ υποβλήθηκαν σε εικονική επέμβαση κρυψορχίας και χρησιμοποιηθήκαν ως ομάδα ελέγχου. Επίσης χρησιμοποιήθηκαν είκοσι ενήλικα θηλυκά κουνέλια τις ιδίας φυλής για τη δοκιμασία γονιμότητας. Την 119η ημέρα της πειραματικής φάσης έγινε η δοκιμασία γονιμότητας κατά την οποία δέκα αρσενικά κουνέλια από κάθε ομάδα επιλεγμένα με τυχαίο τρόπο τοποθετήθηκαν το κάθε ένα ξεχωριστά μαζί με ένα θηλυκό στον ίδιο κλωβό για 24 ώρες, με σκοπό την επίτευξη της γονιμοποίησης. Την 120η ημέρα της πειραματικής φάσης έγινε λήψη δειγμάτων φλεβικού αίματος των δύο ομάδων και μετρήθηκαν τα περιφερικά επίπεδα της τεστοστερόνης. Την 121η ημέρα της πειραματικής φάσης πραγματοποιήθηκε δοκιμασία πρόκλησης τεστοστερόνης με ενδομυϊκή ένεση 5000 U ανθρώπινης χοριονικής γοναδοτροπίνης (hCG) και μετά από τρεις (3) ώρες επανεκτιμήθηκαν τα περιφερικά επίπεδα της τεστοστερόνης ορού του φλεβικού αίματος. Την 122η ημέρα της πειραματικής φάσης τα πειραματόζωα θυσιάστηκαν προκειμένου να αφαιρεθούν οι όρχεις. Το κάθε όργανο, παρασκευάσθηκε προσεκτικά με τη βοήθεια μικροσκοπίου και ζυγίστηκε σε ζυγό ακριβείας. Πραγματοποιήθηκε κατάλληλη έγκληση δείγματος ιστών από το κάθε όργανο προκειμένου να μελετηθούν τυχών αλλοιώσεις του στο οπτικό μικροσκόπιο. Την 144η ημέρα του πειραματισμού τα θηλυκά πειραματόζωα θυσιάστηκαν προκειμένου να ελεγχθεί η μήτρα ως προς την ύπαρξη ή μη εμβρύων. Οι δεξιοί και αριστεροί όρχεις των πειραματόζωων της ομάδας Ι είχαν στατιστικά σημαντικά μικρότερο βάρος από τους αντίστοιχους δεξιούς και αριστερούς όρχεις της ομάδας ελέγχου. Επιπλέον, η μέση διάμετρος των σπερματικών σωληναρίων, το μέσο εμβαδό των σπερματικών σωληναρίων, ο μέσος αριθμός των στρογγυλών σπερματίδων ανά σπερματικό σωληνάριο και ο μέσος αριθμός πρωτογενών σπερμοκυττάρων ανά σπερματικό σωληνάριο στην ομάδα Ι ήταν στατιστικά σημαντικά χαμηλότεροι από ότι στην ομάδα ΙΙ. Επίσης, τα βασικά επίπεδα τεστοστερόνης και η έκκριση τεστοστερόνης μετά από πρόκληση με hCG ήταν στατιστικά σημαντικά χαμηλότερες στα πειραματόζωα της ομάδας Ι από ότι στα πειραματόζωα της ομάδας ελέγχου. Τέλος, τα πειραματόζωα της ομάδας Ι είχαν στατιστικά σημαντικά χαμηλότερη γονιμοποιητική ικανότητα από ότι τα πειραματόζωα της ομάδας ΙΙ. Συμπερασματικά, η χειρουργική πρόκληση μονόπλευρης κρυψορχίας σε ενήλικα κουνέλια είχε σαν αποτέλεσμα την πρόκληση έντονων αλλοιώσεων στον ενδοκοιλιακό όρχη αλλά και την πρόκληση πιο ήπιων αλλά ωστόσο στατιστικά σημαντικών αλλοιώσεων στον ετερόπλευρο οσχεϊκό όρχη σε σύγκριση με τα ζώα της ομάδας ελέγχου. Εκτός από τις ούτως ή άλλως καλά τεκμηριωμένες αλλοιώσεις που εντοπίστηκαν στον ενδοκοιλιακό όρχη στην παρούσα εργασία καταφέραμε να αποδείξουμε την ύπαρξη σημαντικών αλλοιώσεων και στον ετερόπλευρο οσχεϊκό όρχη. Το γεγονός αυτό εξηγεί την μειωμένη γονιμοποιητική ικανότητα των μονόπλευρα κρυψορχικών κουνελιών σε σχέση με την ομάδα ελέγχου
Estimation of the effect of EGR heat on a rankine exhaust heat recovery system from heavy duty diesel truck engines
No full text86 σ.Εκτίμηση της επίδρασης της θερμότητας του ανακυκλοφορούντος καυσαερίου σε σύστημα ανάκτησης θερμότητας μέσω κύκλου Rankine από κινητήρες ντήζελ βαρέων οχημάτωνEstimation of the effect of EGR heat on a Rankine exhaust heat recovery system from heavy duty diesel truck engines.Φώτιος Χ. Δημητριάδη
Empirical Treatments for Male Infertility: A Focus on Lifestyle Modifications and Medicines
No full textBackground/Objectives: Male infertility is a prevalent issue impacting numerous couples worldwide. This review aims to evaluate the effectiveness of empirical therapies for male infertility, focusing on both lifestyle modifications and medical treatments. This study provides a comprehensive overview of interventions aimed at improving male fertility outcomes. Methods: A thorough review of the existing literature was conducted, encompassing studies on lifestyle changes such as dietary changes, smoking cessation, alcohol moderation, and exercise. Additionally, medical treatments including selective estrogen receptor modulators, gonadotropins, aromatase inhibitors, phosphodiesterase-5 inhibitors, antioxidants, dopamine agonists, kallikrein, indomethacin, low-dose corticosteroids, alpha-blockers, and nitric oxide donors were evaluated. The study population included males diagnosed with infertility, focusing on various underlying causes. Results: Lifestyle modifications were found to have a positive impact on sperm quality. Evidence shows that a healthy diet, smoking cessation, moderate alcohol consumption, and regular exercise improve fertility outcomes. Medical treatments demonstrated significant improvements in sperm production and quality. Selective estrogen receptor modulators and gonadotropins enhanced sperm parameters. Aromatase inhibitors and phosphodiesterase-5 inhibitors specifically improved sperm motility and increased pregnancy rates. Antioxidants, such as vitamins E and C and coenzyme Q10, reduced oxidative stress and enhanced sperm counts, motility, and morphology. Dopamine agonists, particularly cabergoline, normalized prolactin levels and improved fertility outcomes. Kallikrein therapy improved sperm parameters and increased pregnancy rates. Indomethacin treatment was associated with increased sperm concentrations and motility. Low-dose corticosteroids and alpha-blockers showed variable results, and nitric oxide donors like L-arginine enhanced sperm counts and motility. Conclusions: Empirical therapies, including lifestyle modifications and medical treatments, significantly enhance sperm quality and reproductive potential. These integrated approaches are essential in improving fertility outcomes in males. However, further extensive randomized trials are necessary to definitively establish the most effective treatments
Antioxidant Supplementation on Male Fertility—A Systematic Review
No full textOur aim was to review the current literature regarding the effect of antioxidant supplementation (AS) on male fertility parameters, as AS is commonly used to treat male infertility due to the availability and affordability of antioxidants in many parts of the world. Materials and methods: PubMed, Medline, and Cochrane electronic bibliographies were searched using the modified Preferred Reporting Items for Systemic Reviews and Meta-Analyses (PRISMA) guidelines to evaluate studies on the benefit of antioxidant therapy on infertile men. Results were analyzed regarding the following aspects: (a) ingredient and dose; (b) potential mechanism of action and rationale for use; and (c) effect on various reported outcomes. Results: Thus, 29 studies found a substantial positive effect of AS on outcomes of assisted reproductive therapy (ART), WHO semen parameters, and live-birth rate. Carnitines, Vitamin E and C, N-acetyl cysteine, coenzyme Q10, selenium, zinc, folic acid, and lycopene were beneficial ingredients. Nevertheless, some studies did not show a substantial change in one or more factors. Conclusion: AS seems to have a positive effect on male fertility. Environmental factors may play an increasing role in fertility. Further studies are needed to determine the optimal AS combination and the influence of environmental factors
Impact of renal sympathetic denervation on cardiac magnetic resonance-derived cardiac indices in hypertensive patients - A meta-analysis
No full textBackground: Renal sympathetic denervation (RDN) is a safe device-based option for the treatment of hypertension although current guidelines do not recommend its use in routine clinical practice. In this meta-analysis, we investigated the effects of RDN in cardiac magnetic resonance (CMR)-derived cardiac indices. Methods: This meta-analysis was performed in accordance with the PRISMA statement. A comprehensive systematic search of MEDLINE database and Cochrane library through to January 2021 was performed. The inclusion criteria were studies that enrolled patients undergoing RDN in whom CMR data were provided for left ventricular end-diastolic volume indexed to body surface area (BSA) (LVEDVI), left ventricular end-systolic volume indexed (LVESVI), left ventricular mass indexed (LVMI), and left ventricular ejection fraction (LVEF) pre and post RDN. A random effects model was used for the analyses. Results: Our search strategy revealed 9 studies that were finally included in the meta-analysis (n = 300 patients, mean age: 60 years old, males: 59%). Compared to control group, RDN patients showed significantly lower values in the attained volumes (LVEDVI:-6.70 ml/m(2), p = 0.01; LVESVI:-3.63 ml/m(2), p = 0.006). Moreover, RDN group achieved a statistically significant higher attained LVEF (3.49%, p = 0.01). A non-significant difference was found in the attained LVMI between RDN and control groups (-2.59 g/m(2), p = 0.39). Compared to pre-RDN values, RDN reduces significantly the LVMI, the LVEDVI, and the LVESVI while a non-significant change of LVEF was found. Conclusions: In conclusion, the current study demonstrates the potential beneficial role of RDN in CMRderived cardiac indices that reflect adverse remodeling. However, large, randomized studies are needed to elucidate the role of RDN in cardiac remodeling in hypertension, heart failure, and other clinical settings. (C) 2021 Japanese College of Cardiology. Published by Elsevier Ltd. All rights reserved
Reconstructing patterns of domestication in reindeer using 3D muscle attachment areas
No full textAbstract
The use of reindeer has been a crucial element in the subsistence strategies of past Arctic and Subarctic populations. However, the spatiotemporal occurrence of systematic herding practices has been difficult to identify in the bioarchaeological record. To address this research gap, this study proposes a new virtual anthropological approach for reconstructing habitual physical activity in reindeer, relying on the protocols of the “Validated Entheses based Reconstruction of Activity” (VERA) method. Following blind analytical procedures, we focused on eight muscle attachment sites (“entheses”) in 36 reindeer free ranging in the wild, 21 specimens in captivity (zoo), and eight racing reindeer (habitual runners). Importantly, our analyses accounted for the effects of variation by subspecies, sex, age, and estimated body size. Our results showed clear differences across activity groups, leading to the development of discriminant function equations with cross-validated accuracies ranging from approximately 88 to 100%. The reliability of our functions was additionally confirmed using a blind test involving six zoo individuals not included in the initial dataset. Our findings support the use of the proposed approach for identifying domestication-related activities in zooarchaeological contexts, introducing a valuable tool for locating suspected domestication hotspots and elucidating the nature of past human-reindeer interactions.Abstract
The use of reindeer has been a crucial element in the subsistence strategies of past Arctic and Subarctic populations. However, the spatiotemporal occurrence of systematic herding practices has been difficult to identify in the bioarchaeological record. To address this research gap, this study proposes a new virtual anthropological approach for reconstructing habitual physical activity in reindeer, relying on the protocols of the “Validated Entheses based Reconstruction of Activity” (VERA) method. Following blind analytical procedures, we focused on eight muscle attachment sites (“entheses”) in 36 reindeer free ranging in the wild, 21 specimens in captivity (zoo), and eight racing reindeer (habitual runners). Importantly, our analyses accounted for the effects of variation by subspecies, sex, age, and estimated body size. Our results showed clear differences across activity groups, leading to the development of discriminant function equations with cross-validated accuracies ranging from approximately 88 to 100%. The reliability of our functions was additionally confirmed using a blind test involving six zoo individuals not included in the initial dataset. Our findings support the use of the proposed approach for identifying domestication-related activities in zooarchaeological contexts, introducing a valuable tool for locating suspected domestication hotspots and elucidating the nature of past human-reindeer interactions
Genetic analysis of over half a million people characterises C-reactive protein loci
No full textData availability: The summary statistics of the CHARGE CRP GWAS used in this study is publicly available from the IEU open GWAS project accession code ieu-b-35 (Trait: C-Reactive protein level - IEU Open GWAS project (mrcieu.ac.uk)). The derived CRP GWAS meta-analysis summary statistics generated in this study has been deposited in the GWAS catalogue under accession code GCST00186 (https://www.ebi.ac.uk/gwas/). Human genome assembly GRCh37 (hg19) from Genome Reference Consortium https://www.sanger.ac.uk/data/genome-reference-consortium/).Copyright © The Author(s) 2022. Chronic low-grade inflammation is linked to a multitude of chronic diseases. We report the largest genome-wide association study (GWAS) on C-reactive protein (CRP), a marker of systemic inflammation, in UK Biobank participants (N = 427,367, European descent) and the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium (total N = 575,531 European descent). We identify 266 independent loci, of which 211 are not previously reported. Gene-set analysis highlighted 42 gene sets associated with CRP levels (p ≤ 3.2 ×10−6) and tissue expression analysis indicated a strong association of CRP related genes with liver and whole blood gene expression. Phenome-wide association study identified 27 clinical outcomes associated with genetically determined CRP and subsequent Mendelian randomisation analyses supported a causal association with schizophrenia, chronic airway obstruction and prostate cancer. Our findings identified genetic loci and functional properties of chronic low-grade inflammation and provided evidence for causal associations with a range of diseases.UK Dementia Research Institute at Imperial College, which receives its funding from UK DRI Ltd. (funded by the UK Medical Research Council, Alzheimer’s Society and Alzheimer’s Research UK) and the British Heart Foundation Centre for Research Excellence at Imperial College London and the National Institute for Health Research Imperial Biomedical Research Centre, Imperial College London. S.S. received funding from the Medical Research Council – Public Health England (MRC-PHE) Centre for Environment and Health awarded studentship, of which funding is derived from the MRC Industrial Strategy Fund. I.T and F.K. have received funding from the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under grant agreement No 1312. R.P. holds a fellowship supported by Rutherford Fund from Medical Research Council (MR/R0265051/1 and MR/R0265051/2). V.K. is funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant (721567)
Effects of a 12-Month Treatment with Glucagon-like Peptide-1 Receptor Agonists, Sodium-Glucose Cotransporter-2 Inhibitors, and Their Combination on Oxidant and Antioxidant Biomarkers in Patients with Type 2 Diabetes
No full textImbalance between oxidative stress burden and antioxidant capacity is implicated in the course of atherosclerosis among type 2 diabetic patients. We addressed the effects of insulin, glucagon-like peptide-1 receptor agonists (GLP1-RA), sodium-glucose cotransporter-2 inhibitors (SGLT-2i), and their combination on levels of oxidant and antioxidant biomarkers. We recruited a total of 160 type 2 diabetics, who received insulin (n = 40), liraglutide (n = 40), empagliflozin (n = 40), or their combination (GLP-1RA+SGLT-2i) (n = 40). We measured at baseline, at 4 and at 12 months of treatment: (a) Thiobarbituric Acid Reactive Substances (TBARS), (b) Malondialdehyde (MDA), (c) Reducing Power (RP), (d) 2,2¢-azino-bis-(3-ethylbenzthiazoline-6-sulphonic acid) radical (ABTS) and (e) Total Antioxidant Capacity TAC). Dual treatment resulted in significant improvement of TBARS, MDA, and ABTS at four months compared with the other groups (p p p p p p < 0.05).12-month treatment with GLP1-RA and SGLT2i resulted in reduction of biomarkers responsible for oxidative modifications and increase of antioxidant biomarker, respectively. The combination treatment was superior and additive to each separate agent and also the beneficial effects appeared earlier
The effect of the COVID pandemic lockdown measures on surgical emergencies: experience and lessons learned from a Greek tertiary hospital
No full textBackground: The COVID-19 pandemic caused a rise in healthcare demands leading to significant restructuring of hospital emergency departments worldwide. The aim of the present study is twofold: firstly, to discern any differences in regard to reason for surgical emergency department (SED) attendance and hospital admission during the pandemic and pre-pandemic eras in Greece, and secondly, to assess the impact of the lockdown measures implemented during the pandemic on SED patient attendance. Methods: Since the beginning of the COVID-19 pandemic in Greece (1 March 2020) and up to 15 December 2020, the charts of all adult patients arriving at the SED of the third surgical department of the “Attikon” University Hospital (a tertiary referral center for surgical and COVID-19 cases) were retrospectively reviewed and broken down in four periods reflecting two nationwide lockdown (period A 1/3/2020 to 30/4/2020 and period D 16/10/2020 to 15/12/2020) and two interim (period B 1/5/2020 to 15/6/2020 and period C 15/9/2020 to 30/10/2020) periods. Demographic and clinical data were compared to those obtained from the same time periods of the year 2019. Results: The total number of patients attending the SED decreased by 35.9% during the pandemic (from 2839 total patients in 2019 to 1819 in 2020). During the first lockdown, there was statistically significant reduction of motor vehicle accidents (p=0.04) and torso injuries (p=0.01). Contrarily, the rate of head injuries (p<0.001) and abdominal pain (p=0.04) were significantly increased. The same effect was observed regarding the rate of hospital admissions (p=0.002), although in terms of absolute numbers, admissions remained unchanged. During the second lockdown, there was a reduction in the number of perianal abscess cases (p=0.04) and hernia-related problems (p=0.001). An increase in the rate of fall injuries was also demonstrable (p=0.02). Overall, application of the lockdown led to a significant decrease in minor (p<0.001) and torso (p=0.001) injuries. Conclusion: The burden of the new COVID-19 disease has left a noticeable imprint in the function of emergency departments worldwide. In Greece, SED attendance was significantly reduced during the pandemic, an effect that was even more pronounced during the lockdown implementation; nevertheless, the overall rate of hospital admissions remained the same, denoting that patient care was not altered. © 2021, The Author(s)
Growth of single-wall carbon nanotubes by chemical vapor deposition for electrical devices
No full textCarbon emerges in di®erent forms. Diamond and graphite have been well known mate-
rials for centuries. Moreover fullerenes and nanotubes were discovered only a few years ago.
H. W. Kroto et al. depicted the fullerenes in 1985 [1]. A few years later, in 1991, S. Iijima
described carbon nanotubes (CNTs) for the ¯rst time [2] (Figure 1.1).
CNTs have a close relation to graphite, since a single-wall carbon nanotube is like a rolled-up
graphite mono layer. However a nanotube has with its curved shape a higher chemical reacti-
vity than a °at graphite layer. Both the side wall and the caps can be modi¯ed chemically [3].
Carbon nanotubes are regular carbon clusters with attractive mechanical and electronic pro-
perties [4]. Nanotubes have a high mechanical strength due to a very large Young's modu-
lus [5]. They can be used for the storage of hydrogen [5, 6], to store energy in electrochemical
double layer capacitors [7] or to reinforce composite materials [3]. A single nanotube can be
used as a sensor [8{12], a nanorelay [13], a vessel [14] or as a template [3, 15]. It is possible
to produce light bulbs [16] and ¯bers [17] with carbon nanotubes. An array of CNTs can act
as a °at panel display [3, 5] using their feature to act as ¯eld emitting devices [18{21].
CNTs are either metallic (1/3) or semiconducting (2/3). Nowadays it is not possible to select
the desired characteristic of a nanotube in advance. It is only possible to separate metallic
from semiconducting tubes by using an electrical ¯eld [22]. Metallic nanotubes with their
diameter of a few nm represent the ultimate conducting wire whereas the semiconducting
ones can be used as transistors [23{25] even on a transparent and °exible substrate [26]. The
transistors can be optimized by the chemical control of the nanotube-electrode interface [27].
Quantum dots [28, 29] and spin valves [30{32] can be built alike simple logic gates [33] and a
Y-junction recti¯er [34].
Carbon nanotubes have a very interesting property: they are "1-dimensional" molecules [35].
This has to be explained in a few words. In general, quantum con¯nement leads to a spacing
of the allowed eigenenergies. Electrons cannot hop into a higher energy level if the thermal
energy is much smaller than this energy di®erence. In a nanotube an electron is con¯ned in the
directions perpendicular to the tube axis. The nanotube becomes a 1-dimensional conductor.
For several years members of our research group are exploring the electrical properties of
this very special conductor. The behavior of carbon nanotubes is investigated with electrical
transport measurements at low temperatures (down to 50 mK) and in high magnetic ¯elds
(up to 10 T).
The raw material for the ¯rst experiments [36{38] were multi-wall carbon nanotubes ob-
tained from L. Forr¶o (Ecole Polytechnique F¶ed¶erale de Lausanne) which were produced using
laser ablation. The multi-wall carbon nanotubes were used to investigate the suppression of
tunnelling [36, 39], multiple Andreev re°ection [28, 37], electrical spin injection [30{32] and
quantum dots [37, 40{43].
The next step was to grow single-wall carbon nanotubes using chemical vapor deposition
(CVD) [8,44{46]. This procedure has the advantage to be faster than an external collaboration
and in addition the growth of the tubes directly on the device makes the samples ready for
use without an additional treatment.
It was veri¯ed that the CVD grown tubes are suitable of for electrical devices [47]. Vibrating
nanotubes [48] and an ambipolar ¯eld-e®ect transistor [23] were studied. Kondo e®ect [49]
and Fano-Resonances [50] were investigated as well.
The latter experiments reveal one common de¯ciency. The grown tubes are often not sepa-
rated but bundled [47] (Figure 6.10). Moreover it is not clear if they are multi- or single-wall
tubes. This means for electronic transport measurements that several tubes are measured si-
multaneously. Thus the tube with the best conductivity dominates the measurement, whereas
the other tubes perturb the measured signal by there presence.
The main focus of this thesis is the development of a growth process of single-wall carbon
nanotubes by using CVD. The aim is to overcome the problem of bundling. The grown
nanotubes have to be free of lattice defects and they need to have good electrode-nanotube
contacts in order to make them suitable for electronic transport measurements. They have
to lay °at, well separated and optimally distributed on SiO2 our standard substrate. On the
one hand the tube density should not be too high since this would increase the probability
of shortcuts between the electrodes due to nanotube-nanotube contacts. On the other hand
it should not be too low since this would make the localization of an appropriate nanotube
much more time consuming (Figure 1.2).
Two ways to achieve this goal were tried. The single-wall nanotubes can be bought, dissolved
in a solvent and spread after cleaning and separation [51{57], as in the thesis [46]. The second
possibility is to grow the tubes directly on the device as presented in this thesis.
Growing carbon nanotubes with CVD is very simple, at least in principle. There are only
a few essential things needed: an oven, a substrate, a catalyst and a carbon feedstock. The
main challenge is to acquire the right knowhow.
The ¯rst step was to build up the CVD system. Afterwards the proper growth conditions
and a simple method to check the demanded properties of the grown tubes had to be found.
Scanning electron microscopy (SEM) is the standard characterization tool used in this thesis.
Transmission electron microscopy (TEM) is a helpful mean in order to show that the tubes
are separated and single-wall, since it allows the investigation of the tubes' internal structure.
Atomic force microscopy (AFM) and Raman spectroscopy are used in addition.
Outline of this thesis
² Chapter 2 gives a short overview with respect to the properties, the growth and the
characterization of carbon nanotubes.
² The oven and the gas system are delineated in Chapter 3. Di®erent carbon feedstocks
were used: ethylene/hydrogen, methane, methane/ethylene and methane/hydrogen.
² The steps towards a suitable catalyst are presented in Chapter 4. Evaporated and
liquid based catalysts were tested. An iron molybdenum alumina catalyst dissolved in
2-propanol provides the best results.
² Chapter 5 gives a comparison of the results obtained utilizing di®erent growth processes,
and describes the formation of amorphous carbon and the oxidation of nanotubes.
² Chapter 6 summarizes experiments on di®erent TEM grids (Au, Cu, Mo, Ni, stainless
steel, Ti, quantifoils) and silicon nitride windows.
² The results from collaborations with other group members are presented in Chapter 7.
These experiments show the good quality of the grown tubes
