Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals

Abstract

Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir-Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10(3) was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10(4). This is an important step towards the detection of the phonon spectrum from a single QD.

Morphology-induced phonon spectra of CdSe/CdS nanoplatelets: core/shell vs. core–crown

“…The feature peaked at 250 cm −1 may be related with Se on the surface of CdSe part. 16,27 The broader scattering feature cen-tered near 270 cm −1 can be attributed to CdS SO phonon scattering. The latter mode is expected in the frequency range between bulk CdS TO (240 cm −1 ) and LO (305 cm −1 ) 55 and was repeatedly observed for CdS nanostructures.…”

“…48,49 Recent investigations of CdSe NPLs and spherical NCs by means of surface enhanced Raman scattering additionally suggest a contribution of surface Se to this spectral feature. 27 This could be a plausible assumption for CdSe NCs, because the strongest features of elemental or amorphous Se is known to peak near 250 cm −1 . 16 However, an analogous shoulder was reported for CdS and CdTe NCs, 50 but the Raman features of Te are not above but below the LO feature of CdTe, 51 and the elemental sulphur peaks are also far from the CdS LO position.…”

Efficient SERS substrates based on laterally ordered gold nanostructures made using interference lithography

“…Most of the technologies used to fabricate the ordered nanostructures, such as electron-beam lithography and molecular-beam epitaxy [18], are expensive, making these substrates economically not attractive. Therefore, substrates made using such techniques are produced in limited quantities and are intended solely for scientific research.…”

Optical Plasmon Resonances in Arrays of Au Nanoclusters

“…Однако такой способ требует прозрачной подложки, например кварца, сапфира или стекла. Действительно, большинство работ, исследующих ЛППР, представляют результаты для металлических нанокластеров, размещенных на прозрачной подложке, например [12; 13], только немногие имеют дело с непрозрачным материалом подложки, таким как кремний [14][15][16][17][18]. Определение частотного положения ЛППР для плазмонных структур на непрозрачных подложках является непростой задачей из-за сложности разделения излучения рассеянного плазмонами и обратного отражения от подложки [19].…”

“…Максимумы в расчетных спектрах поглощения плазмонных образцов использованы для определения энергий ЛППР в массивах нанокластеров Au на непрозрачных подложках. Наши результаты согласуются с литературными данными, полученными ранее альтернативными методами для нанокластеров Au идентичной формы на различных подложках [11][12][13][14][15][16][17].…”