Our research
Excitons in Nanoscale Systems
An exciting aspect of nanoscience is that relationships between structure and electronic properties are being revealed through a combination of synthesis, structural characterization, chemical physics, and theory. A challenge in this field is that the materials under investigation are often quite complex; they contain many atoms, they can be structurally disordered or influenced by surface effects, and samples often have an inhomogeneous composition in terms of structural disorder and size polydispersity. Nonetheless, rapid progress has been made in our understanding of a diverse range of materials. Our research contributes to the current understanding of excitons through investigations of various nanostructured materials, with a particular emphasis on elucidating new aspects of exciton photophysics.
Excitons are of great interest in nanoscience because it has been discovered that their properties can be dictated by the size and shape of a material—as well as its constitution—when at least one dimension is smaller than the usual size of the exciton in an infinite-sized material. The new aspect of excitons that is prevalent—or even that defines—nanoscience is that the physical size and shape of the material is a parameter that significantly influences the properties of excitons. That interests us because (a) Excitons can be engineered in a material by the arrangement of building blocks. (b) The spatial compression of the exciton accentuates many of its interesting physical properties; exposing them for examination. (c) Our understanding is challenged: Perhaps we should not try to force-fit existing theories to model excitations in nanostructures as oftentimes they carry with them assumptions that we have forgotten, or that we erroneously ignore. Nano-scale materials thus provide both a test-bed and an inspiration for new quantum mechanical approaches to the calculation of electronic properties of large systems.
The figure shows excitons and structural size variations on the nanometer length scale. (a) The photosynthetic antenna of purple bacteria, LH2, is an example of a molecular exciton. The absorption spectrum clearly shows the dramatic distinction between the B800 absorption band, arising from essentially “monomeric” bacteriochlorophyll-a (Bchl) molecules, and the red- shifted B850 band that is attributed the optically-bright lower exciton states of the eighteen electronically-coupled Bchl molecules [e.g. Scholes & Fleming, Adv. Chem. Phys. 2005. (b) The size-scaling of polyene properties, for example oligophenylenevinylene oligomers, derives from size-limited delocalization of the molecular orbitals. However, as the length of the chains increases, disorder in the chain conformation impacts the picture for exciton dynamics. Absorption and fluorescence spectra are shown as a function of the number of repeat units. [Reproduced courtesy of Dr. Johannes Gierschner.] (c) Single wall carbon nanotube (CNT) size and “wrapping” determine the exciton energies. Samples contain many different kinds of tubes, therefore optical spectra are markedly inhomogeneously broadened. By scanning excitation wavelengths and recording a map of fluorescence spectra, the emission bands from various different CNTs can be discerned, as shown. [The figure was kindly provided by Dr. Marcus Jones.]. (d) Rather than thinking in terms of delocalizing the wavefunction of a semiconductor through interactions between the unit cells, the small size of the nanocrystal confines the exciton relative to the bulk. Size-dependent absorption spectra of PbS quantum dots [Hines & Scholes, Adv. Mater. 2003] are shown.
Reference:
G. D. Scholes & G. Rumbles, Nature Materials (2006) 5, 683–696.
Ultrafast exciton fine structure relaxation in quantum dots
Many spectroscopic investigations of quantum dots (QDs) are undertaken for colloidal samples, which are typically randomly oriented. In such samples the exciton fine structure is obscured by inhomogeneous line broadening, so individual states cannot be selected by tuning the frequency of the excitation light. Polarized excitation light may allow one to probe the fine structure exciton states with greater selectivity. In that case, resonant nonlinear optical spectroscopy has great potential; the higher the order of the spectroscopy, the more diverse the opportunities for using polarization to gather information from an isotropic ensemble.
Recently we have shown how cross linearly-polarized pulse sequences in three-pulse transient grating experiments form a polarization grating that monitors flipping among populations of quantum dot exciton spin states (those with total angular momentum F = +/-1 and +/-2). Spin flips among those states lead to a decay of the grating, and consequently the diffracted probe signal. The experiment was simulated by solving the equations of motion that dictate the temporal evolution of the third-order density matrix. In the microscopic picture elucidated from the simulations, destructive interference between the third-order polarizations radiated by populations of excitons with flipped and conserved spin states causes the signal decay. Hence this ultrafast laser experiment provides a means to detect the history of the quantum dot exciton states and to examine the relaxtion processes among the lowest exciton levels of quantum dots.
The results of new experiments based on heterodyne detection of the radiated polarization demonstrate the theory unambiguously. Three ultrafast laser pulses interact with the sample (a solution or film of QDs). Each pulse n the sequencehas a vertical (V) or horizontal (H) polarization, and the signal, which is radiated in a fourth direction, is detected through an analyzer set to either V or H. Data, reported for a CdSe quantum dot sample and shown in the figure, test the theory by confirming the sign-dependence of the VHHV signal compared to VHVH and VVVV. It was found that exciton relaxation occurs over at least two characteristic tiome scales, depending strongly on the quantum dot size.
References:
(a) Gregory D. Scholes, “Selection rules for probing biexcitons and electron spin transitions in isotropic quantum dot ensembles,” J. Chem. Phys. 121, 10104-10110 (2004).
(b) Vanessa M. Huxter, Vitalij Kovalevskij, & Gregory D. Scholes, “Dynamics within the exciton fine structure of colloidal CdSe quantum dots,” J. Phys. Chem. B. Letters 109, 20060–20063 (2005).
(c) Gregory D. Scholes, Jeongho Kim, & Cathy Y. Wong, “Exciton spin relaxation in quantum dots measured using ultrafast transient polarization grating spectroscopy” Phys. Rev. B (2006) 73, 195325.
(d) Gregory D. Scholes, Jeongho Kim, Cathy Y. Wong, Vanessa M. Huxter, P. Sreekumari Nair, Karolina P. Fritz, & Sandeep Kumar, “Nanoscale shape and the mechanism of exciton spin relaxation ” Nanolett. (2006) 6, 1765–1771.
Nanocrystal shape control: Towards building new nanoscale architectures
Inspiring work from a number of groups in recent years has paved the way to shape-control of colloidal semiconductor nanocrystals. The opportunity we see is that now the role of shape in governing the optical properties of these materials can be explored, and ths is a major thrust of our recent work.
Some materials recently prepared by us are shown in the figure. (a) TEM images of CdSe nanorods with an average aspect ratio of ~1.6. The scale bar corresponds to 50 nm. (b) A high resolution TEM image of an individual CdSe nanorod, looking down the crystal c-axis. (c) and (d) By varying growth conditions, a myriad of other shapes can be prepared by selective growth of different crystal faces. The scale bars each correspond to 20 nm. Note that each end of the rod is not identical, since there is no centre of inversion or reflection symmetry in a plane normal to the c-axis of the crystal, therefore asymmetric rods, like the bullet-shaped colloids seen in (d) can be grown.
References:
(a) P. S. Nair, K. P. Fritz, & G. D. Scholes, "Evolutionary Shape Control During Colloidal Quantum Dot Growth", Small (2007) 3, 481–487.
(b) Sandeep Kumar, Marcus Jones, Shun S. Lo, & Gregory D. Scholes, “Nanorod Heterostructures Showing Photo-induced Charge Separation” Small (2007) 3, 1633–1639.
