RESUMEN
We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.
RESUMEN
HISTORY AND CLINICAL FINDINGS: A 27-year-old woman presented with chronic diffuse bone pain and skeletal deformities. Since the age of 3 years she had occipital hyperostosis. Since aged 13 years she had symptoms indicating spinal root involvement due to hyperkyphosis. For the last 6 years there was evidence of destructive and hyperplastic changes in the region of the middle ribs. Alkaline phosphatase (APL) had been elevated (> 250 U/l) for several years. Three years before the present admission the patient had been hospitalized elsewhere because of bone pain and had received intermittent infusions of pamindronate to a total of 600 mg. INVESTIGATIONS: Laboratory tests showed moderate rise in bone-specific APL and osteocalcin. Conventional x-ray examinations and computed tomography showed lesions in the occipital bone, sella turcica and ethmoid air cells. Several thoracic vertebral fractures were demonstrated. Skeletal scintigraphy indicated enrichment in the occipital and supraorbital regions, as well as in vertebrae and ribs. Dual energy x-ray absorptiometry (DEXA) showed osteopenia. Bone biopsy from the occipital lesion revealed fibrous dysplasia. TREATMENT AND COURSE: The diagnosis of fibrous dysplasia having been confirmed, cyclical treatment with i.v. pamidronate was given for 6 months. It was well tolerated and lessened the pain without changing the objective findings. CONCLUSION: In case of uncertain radiological and scintigraphic findings in the differential diagnosis of fibrous dysplasia, Paget's disease or bone tumor, it is essential to obtain a biopsy and, if available, perform gene analysis.
