Giant and Broad-Band Absorption Enhancement in Colloidal Quantum Dot Monolayers through Dipolar Coupling

Abstract

We demonstrate giant and broadband enhancement of the nanocrystal absorption cross section in close packed nanocrystal superlattices, which is the first report on a collective optical phenomenon in this type of self-assembled metamaterials to date. Colloidal nanocrystals (NC) are nanometer-sized semiconductor crystals obtained through a solution-based chemical synthesis. Apart from changing their composition, the optical properties (such as the absorption cross section and band gap) of the crystals can be tuned by varying their size, a direct result of strong quantum confinement in these structures. Moreover, their easy deposition allows for cheap bottom-up fabrication. For these reasons, nanocrystals are thought to be viable candidates for opto-electronic applications such as the fabrication of solar cells, light emitting diodes, photo-detectors, etc…. In such a device context, nanocrystals will very often be deposited in thin layers. However, as NCs are obtained through wet chemical synthesis, their optical properties (e.g. absorption cross section) are typically evaluated in solution using effective medium approaches, which assume no electromagnetic coupling between the absorbing dipoles. It is questionable that this assumption still holds for a close packed film of nanocrystals where those dipoles tend to couple through electromagnetic multipolar interactions. Using properties measured in solution to evaluate and study the performance and physics of thin film devices would therefore be incorrect, or at least a serious simplification. The basic idea of our approach to understand such close-packed dipoles is that of the 'coupled dipole model' (CD): the internal field of a particle in close proximity to other dipoles will be a superposition of the influence of the external field and the induced dipolar fields of the neighboring particles (see Figure 1, right). To quantify this idea, we will focus on the NC absorption cross section and we define an 'enhancement factor 'E', being the ratio between the absorption cross section in film to the cross section in solution and try to obtain E both through theory (via the coupled dipole model) and experiment 2. Figure 1: Close-up of nanocrystal monolayer (a: Image of monolayer on glass, b: AFM surface morphology, c: TEM image and d: TEM close-up showing local hexagonal order) and schematic of near-field coupling mechanism proposed in this work. To access the enhancement E experimentally, we measure the absorption spectrum of single and multiple close packed monolayers of nanocrystals on glass substrates (see Figure 1) using a standard UV-VIS-NIR spectrophotometer. The layers (essentially a meta-material) are deposited using Langmuir-Blodgett deposition 1 and are well defined (i.e. particle density and lattice symmetry are uniform over large areas). The enhancement predicted by our CD interpretation is substantial, leading to a size-dependent value of E up to 5 and 4 for PbS and CdSe monolayers respectively (see figure 2, left). To test our CD modeling, we use 2D single c...

Synthesis and Optical Properties of Water-Soluble CdTe:Zn2+ Quantum Dots Prepared by the One-Pot Approach

“…In the last few decades, nanostructured materials, such as fullerenes, nanotubes, nanoribbons, nanowires and nanocrystals (NCs), have attracted great attention due to their remarkable optical and electronic properties. In particular, semiconductor NCs, crystalline structures that can be confined in one, two or three spatial dimensions with length varying from 1 to 100 nm, 1,2 present special optical features related to the strong confinement effect of electrons, which occurs when the NC radius becomes smaller than the Bohr radius. 3,4 Materials that exhibit a system of electrons confined in all three spatial dimensions have quantized energy levels, such as atoms and molecules.…”

Energy Transfer in Silicon Nanocrystal Solids Made from All-Inorganic Colloidal Silicon Nanocrystals

“…4,[6][7][8]19 In general, optical properties of NC-based solid structures are different from those of individual NCs due to different dielectric environment of NCs and due to interactions between NCs. Modification of dielectric environment of NCs results in the enhancement of light absorption cross sections 20 and emission rates. 21,22 If NCs are very closely packed and the wave functions are overlapped, formation of mini-bands is expected.…”

Self‐Assembly of Au15 into Single‐Cluster‐Thick Sheets at the Interface of Two Miscible High‐Boiling Solvents

“…If the growth along one specific dimension is suppressed, 2D-oriented growth will be favored. [17][18][19][20][21][22][23][24][25][26][27] Most recently, CdSe NCs have been reported as intermediates for the synthesis of crystalline nanosheets. [8][9][10][11][12][13] Within the soft template of lamellar cadmium compounds, the 2D-limited recrystallization of NCs generates CdSe sheets.…”