One-dimensional chains of sp-hybridized carbon atoms stand at the crossroads between carbon nanostructures and π-conjugated organic systems; therfore they are very appealing from both a fundamental and an applicative point of view. The structure of such carbon atomic wires (CAWs) ranges between two limit bond-patterns: cumulenes, composed by an equalized sequence of double bonds, and polyynes, containing alternating single and triple bonds. The actual configuration of molecular orbitals affects the key electronic and optical properties which can be tuned by varying the chain length and termination. Though the “exotic” nature of CAWs makes them quite unstable species, they have been observed inside several naturally occurring and artificial carbon systems; special termination, indeed, can stabilize the chains opening the way to applications down to the single-molecule level. In this thesis work two series of differently terminated polyynes are characterized by Raman spectroscopy in order to provide information on their structural and electronic properties. Solid-state Surface Enhanced Raman Spectroscopy (SERS) analysis performed on evaporated Ag-nanoislands substrates allows a simpler observation of characteristic features and an effortless evaluation of the effects of chain length, termination and exciting laser wavelength. Furthermore, appearance of new features in SERS spectra reveals an interaction between CAWs and metal nanostructures, able to alter the structure from polyynic to cumulenic. This observation validates the hypothesis of a charge transfer occurring from the nanoislands to the chains, as proposed in literature for analogue systems; molecular geometry-induced deviations from this behavior are witnessed for the first time, too. Further research on this phenomenon could lead to the development of CAWs-based devices with switchable electronic properties.
I fili atomici di carbonio (CAW, dall’inglese ‘carbon atomic wires’), collocandosi al crocevia tra le nanostrutture di carbonio e i sistemi molecolari coniugati, risultano materiali molto interessanti da un punto di vista sia scientifico di base sia applicativo. La struttura di queste catene monodimensionali di atomi di carbonio ad ibridizzazione sp può variare tra due configurazioni-limite dei legami: i cumuleni, composti da una sequenza equalizzata di doppi legami, e le poliine, che contengono un’alternanza di legami singoli e tripli. La conformazione effettiva degli orbitali molecolari influisce sulle principali proprietà elettroniche ed ottiche, le quali possono essere regolate variando la lunghezza e la terminazione della catena. Nonostante la singolare struttura le renda specie piuttosto instabili, alcune catene ad ibridazione sp sono state osservate all’interno di parecchi sistemi di carbonio naturali o artificiali; particolari terminazioni possono infatti stabilizzare i fili atomici, aprendo così la strada a molteplici applicazioni fino al livello di singola-molecola. In questo elaborato di tesi due serie di poliine con diversi gruppi terminali sono caratterizzate tramite spettroscopia Raman al fine di ricavare informazioni sulle loro proprietà strutturali ed elettroniche. La Spettroscopia Raman Amplificata da Superfici (acronimo inglese: SERS), realizzata mediante substrati solidi con nanoisole di argento evaporate, consente una più semplice misurazione dei segnali caratteristici rendendo così possibile un’agevole valutazione degli effetti indotti dalle differenti lunghezze di catena, terminazioni e frequenze eccitatrici. Inoltre, la comparsa di una nuova banda negli spettri SERS rivela l’esistenza di un’interazione tra le nanostrutture metalliche e i CAW in grado di variarne la struttura, rendendola cumulenica. Tale riscontro avvalora l’ipotesi di un trasferimento di carica dalle nanoisole alla catena, già proposta in letteratura per sistemi simili; deviazioni da questo comportamento indotte da variazioni nella geometria molecolare sono inoltre osservate per la prima volta. Un proseguimento della ricerca su questo fenomeno potrebbe pertanto condurre allo sviluppo di dispositivi basati su CAW con proprietà elettroniche convertibili.
Vibrational and structural properties of carbon atomic wires investigated by Raman and surface enhanced Raman spectroscopy
BARBIERI, VALENTINO
2013/2014
Abstract
One-dimensional chains of sp-hybridized carbon atoms stand at the crossroads between carbon nanostructures and π-conjugated organic systems; therfore they are very appealing from both a fundamental and an applicative point of view. The structure of such carbon atomic wires (CAWs) ranges between two limit bond-patterns: cumulenes, composed by an equalized sequence of double bonds, and polyynes, containing alternating single and triple bonds. The actual configuration of molecular orbitals affects the key electronic and optical properties which can be tuned by varying the chain length and termination. Though the “exotic” nature of CAWs makes them quite unstable species, they have been observed inside several naturally occurring and artificial carbon systems; special termination, indeed, can stabilize the chains opening the way to applications down to the single-molecule level. In this thesis work two series of differently terminated polyynes are characterized by Raman spectroscopy in order to provide information on their structural and electronic properties. Solid-state Surface Enhanced Raman Spectroscopy (SERS) analysis performed on evaporated Ag-nanoislands substrates allows a simpler observation of characteristic features and an effortless evaluation of the effects of chain length, termination and exciting laser wavelength. Furthermore, appearance of new features in SERS spectra reveals an interaction between CAWs and metal nanostructures, able to alter the structure from polyynic to cumulenic. This observation validates the hypothesis of a charge transfer occurring from the nanoislands to the chains, as proposed in literature for analogue systems; molecular geometry-induced deviations from this behavior are witnessed for the first time, too. Further research on this phenomenon could lead to the development of CAWs-based devices with switchable electronic properties.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/106842