multi-electron H 2 O and CO 2 activation, but
methods are lacking that allow coupling of these
components to electron/hole conducting moieties
in 3-D frameworks. Molecular-type linkages
need to be developed for efficient charge
conduction between catalytic sites and
photoactive components embedded in the
assembly.
Integrated Time-resolved Probes
Current research on self-assembly has been
limited to observation of ordered structures using
conventional techniques such as X-ray and
electron diffraction, transmission electron
microscopy, and atomic force microscopy.
Moving self-assembly science forward requires
an experimental window that reveals the three-dimensional structural nature, and time scales of
the “embryonic nuclei” that trigger self-assembling processes as they cross from the nanoscale to
microscopic and macroscopic dimensions. It is equally critical to observe in real time and space
the transformations and intermediate states that assemblies go through before reaching their final
form. This information is not presently accessible, and requires invention of “integrated” time-
resolved probes that record in real time the evolution of the system across length scales. These
might be presently unknown hybrids of scanning probe techniques, near-field strategies, confocal
microscopy, magnetic resonance imaging, tomographic techniques, vibrational spectroscopies,
and others. Opening this spatial and temporal window on self-assembling systems will allow us
to direct systems externally (e.g., through solvent, temperature, external fields, and photons) into
the desired targets. A grand challenge is to develop such probes for “self-assembly dynamics”
that tolerate compositionally controlled atmospheres, liquid phases, variable temperature, and
variable pressure.
New Computational Approaches
New computational approaches are needed to integrate simulations across disparate time and
length scales that are important for assembly of solar fuel/energy producing systems (see
Figure 65). For example, modeling has traditionally been carried out separately for increments of
length scales using quantum mechanics (0.1–10 nm), statistical mechanics (1–1,000 nm),
mesoscale (0.1–100 μm), and continuum mechanics (1 mm–10 m). Time scales range from
quantum mechanical methods (10−^15 s) to continuum methods (1–10^5 s). There is a critical need
for theoretical modeling and simulation (TMS) to span all these length and time scales
seamlessly to meet the needs of solar research, to provide insight into the forces and processes
that control the organization of functional elements over all length and time scales; to understand
quantitatively the kinetics of catalyzed photochemical energy conversion reactions over many
length scales in complex, hybrid systems; to identify active sites on nanostructured surfaces, etc.
Figure 64 Hierarchical assembly of
mesoporous oxide within nanoscopic channels
of porous alumina membrane. This example
illustrates principles of multiple length
ordering.