Nanoszerkezetek és alkalmazott spektroszkópia csoport

A csoport munkatársai


A csoport gazdag tapasztalatokkal rendelkezik az amorf vékonyrétegek elõállítása és vizsgálata területén. A kutatások általában arra irányultak, hogy felderítsék az összefüggéseket a szerkezet és olyan makroszkópikus tulajdonságok között, mint az elektromos transzport, az optikai sajátságok, a fotovezetés és a fotolumineszcencia. Mind az anyagminták elõállításához, mind pedig a kísérleti vizsgálatokhoz szükséges berendezések többségét a csoport mûködteti.

Az utóbbi néhány évben az amorf szén vékonyrétegekkel és fõként a gyémántszerû tulajdonságokat mutató filmekkel foglalkoztunk. Rádiófrekvenciás (rf) ködfénykisüléssel állítjuk elõ ezeket a rétegeket különbözõ szerves anyagokból (metán, benzol, stb) a gázösszetétel, áramlási sebesség és gáznyomás, valamint az rf teljesítmény, mint paraméterek szabályozásával. Így az elõállított rétegek tulajdonságai széles tartományban kontrolláltan változtathatók.

Az így elõállított minták atomi skálájú szerkezetét Raman szórással és infravörös spektroszkópiával határozzuk meg. A rétegek elektromos vezetõképessége a szigetelõtõl a félvezetõn keresztül a félfémesig változtatható, ezért fontos az elektromos vezetés mechanizmusának megértése ezen filmekben. Az egyenáramú vezetés hõmérsékletfüggésének mérésével, illetve a vezetési fluktuációk vizsgálatával következtetünk a domináns vezetési mechanizmusokra. A nagy tilos sávszélességû amorf szén vékonyrétegek hatékony szobahõmérsékletû lumineszcenciát mutatnak, ami a lapos kijelzõk fejlesztéséhez járulhat hozzá számottevõen. Ugyanezen fejlesztések szempontjából fontos, hogy e nagy tilos sávszélességû szénrétegek elektron kilépési munkája igen kicsi.

Ezen amorf szén vékonyrétegek elõállítása során megfelelõ elõállítási paraméterek mellett a fémes hordozó felületén nagy hatékonysággal képzõdnek azok a szén szerkezetek, amelyeket grafit nano-csöveknek neveznek. E csövek valószínûleg más esetekben is kialakulhatnak kisebb koncentrációban, így vizsgálatainkat a jövõben kiterjesztjük ebbe az irányba is. Ezen nano-csövek nagy kihívást jelentenek a kutatás számára, és nagy gyakorlati jelentõségük már most látszik számos területen, mint pl. a hidrogén tárolás.

További információk

Veres Miklós, e-mail:

A csoport legfrissebb eredményei (angolul):

Silicon-vacancy center in nanodiamonds. – Due to their excellent light emission properties, the investigation of the negatively charged silicon–vacancy (SiV) centers in diamond nanostructures has attracted much interest during last decades. The SiV center consisting of one interstitial silicon atom in a split–vacancy configuration with D3d symmetry has a bright, stable and narrow zero-phonon line (ZPL) at 1.681 eV. It has a weak phonon sideband since more than 70% of the emitted light is concentrated into the ZPL even at room temperature. These advantageous properties of the optical transition make the SiV center a promising candidate for solid-state single-photon emitter that can be used to realize numerous novel applications in quantum computing and cryptography or nanoscopy and cell biology, etc. For quantum information and quantum processing purposes, indistinguishable single photons from distinct SiV center emitters are required. Variation of spectral parameters of the mentioned center observed by different groups strongly restricts its implementation to real applications. Novel methods have to be developed for the creation of uniformly emitting silicon–vacancy centers in diamond nanostructures on large scale, and the origin of the processes responsible for the variation of the ZPL of the SiV centers has to be resolved.

Figure 1. Spectral distribution of SiV center emission line in photo­luminescence spectra measured at room temperature in nanocrystalline diamond layers grown at substrate temperatures and methane concentrations in the precursor Ar + CH4 mixture of (1) T=700 oC and 2.0% CH4 (2) T=750 oC and 0.2% CH4 (3) T=750 oC and 3.0% CH4 (4) T=800 oC and 1.0% CH4 (5) T=850 oC and 0.2% CH4 (6) T=850 oC and 3.0% CH4. The emission was excited by the 2.54 eV line of an Ar-ion laser.

In order to have detailed overview on the spectral properties, SiV center ensembles were investigated in a large number of nanodiamond films by fluorescence spectroscopy (Fig. 1). Conditions of SiV center formation were varied systematically in microwave plasma-assisted chemical vapor deposition (MW CVD) process and spectral parameters of the zero-phonon line (ZPL) were obtained by fitting procedure from experimentally measured spectra. The average size of nanodiamond grains determined from scanning electron microscopic (SEM) micrographs and residual stress of diamond layer calculated from diamond Raman peak position were used as sample parameters. The SiV centers ZPL peak positions were found to vary from 1.677 to 1.681 eV, while their line broadening between 6.5 and 18.1 meV. The smallest linewidth was observed in a diamond layer of 30 nm average grain size and it was comparable with line broadening values reported for individual SiV centers.

A significant blue shift and line narrowing of the ZPL peak position was observed with decreasing average grain size of the SiV containing diamond thin films. The residual stress, being dependent on the grain size, was identified as the major cause of the variation of the ZPL parameters. It was found that the increase of the residual stress from 0.64 GPa tensile to 2.25 GPa compressive one correlates well with the changes in ZPL peak parameters. The blue shift and line narrowing of SiV centers were explained by the suppression of the orbital relaxation processes, involving ground and excited electronic levels, initiated by the different local strain fields in the vicinity of the SiV centers. Acoustic phonon mode confinement due to small diamond grain size also contributes to the suppression of relaxation process by electron–phonon transitions. Our results indicate decisive influence of the diamond grain size and internal residual stress of the layer on the spectral parameters of the zero phonon line of the SiV centers.

Nickel-silicon (Ni-Si) related complex color center in nanodiamond. — Ni-Si related complex color center was successfully created in nanocrystalline diamond grains through CVD deposition process, which emits highly intensive narrow-bandwidth ZPL at 865 nm (1.433 eV) with 2 nm (3 meV) full width at half maximum. This color center is highly significant in the field of biological and medical applications since its excitation and emission wavelength range is lying in the near-infrared window of biological tissues. Variation of ZPL peak position and line width have been detected in nanodiamond grains prepared under different conditions (Fig. 2). Experimental results on the residual stress determined from the position of diamond Raman peak measured on different nanodiamond grains exhibit different values in the range of -0.963 to +0.284 GPa, and can be of compressive or tensile type. In contrast to the SiV color center, direct relation between the ZPL position and local stress has not been established until now. The relatively large size of the complex center and the lack of vacancy in the center could be the possible reasons of this behavior. (Fig. 2)

Figure 2. Dependence of the ZPL spectral shape of Ni-Si related complex impurity center in diamond nanograins on deposition conditions. The spectra were recorded by 488 nm laser excitation at room temperature.

Preparation of nanocrystalline diamond and nanocrystalline silicon carbide by ultrashort laser pulses

Favorable properties of color centers formed in nanodiamond for various applications in quantum informatics, medical imaging and biolabeling generate a need to develop fast, reliable and widespread technology for their fabrication. Therefore a new concept was introduced for the creation of one-photon emitter centers in nanodiamond by using ultrashort laser pulses (800 nm center wavelength with 1 kHz repetition rate and 42 fs pulse duration). Nanodiamond crystals were produced with ultrashort laser pulses by using different carbon- and silicon-based materials as source material. Surface-enhanced Raman spectroscopy measurements were performed to prove the presence of diamond nanocrystals (Fig. 3, left). The incorporation of silicon foreign atoms into the nanodiamond structure under laser irradiation was also demonstrated.

 

Figure 3.(Left) Surface-enhanced Raman scattering spectrum of nanocrystalline diamond grains. (Right) Micro-Raman spectrum of the silicon carbide agglomerate excited by 514 nm probing wavelength.

Silicon carbide (SiC) nanocrystals were created successfully by the irradiation of carbon- and silicon-based materials with femtosecond laser pulses (800 nm center wavelength with 1 kHz repetition rate and 42 fs pulse duration) in air at room temperature. Micro-Raman spectroscopic and scanning electron microscopy measurements were carried out to prove the presence of SiC nanocrystals and to analyze the bonding structure of formed nanostructures. Detailed analysis of the transversal (TO) and longitudinal (LO) optical modes support the formation of cubic and hexagonal SiC nanocrystals (3C, 4H and 6H) with average grain size of 100-500 nm (Fig. 3, right).