DEC. 17, 2015

Berkeley Lab researchers model hot carrier movement in real-time.

Plasmons, which may be thought of as clouds of electrons that oscillate within a metal nanocluster, could serve as antennae to absorb sunlight more efficiently than semiconductors. Understanding and manipulating them is important for their potential use in photovoltaics, solar cell water splitting, and sunlight-induced fuel production from CO2.
But in these applications, single particle excitation rather than the collective plasmon excitation is needed to transfer electrons one at a time to an electrode and induce desired chemical reactions. After the plasmon is excited by sunlight, it induces the single particle excitation ‘hot carriers.’ Now, for the first time, the interplay between the plasmon mode and the single particle excitation within a small metal cluster has been simulated directly.

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) used a real-time numerical algorithm, developed at Berkeley Lab in February, to study both the plasmon and hot carrier within the same framework. That is critical for understanding how long a particle stays excited, and whether there is energy backflow from hot carrier to plasmon. The new study shows the electron movement when it is perturbed by light.
‘’You need to consider how the plasmon can give its energy to single particle excitations. People have done this analytically, but they looked at the bulk-like material and treated the plasmon mode using classical description,’’ says Lin-Wang Wang, senior staff scientist at Berkeley Lab, who led this work. ‘’We have described both the plasmon and the single particle excitation quantum mechanically, and studied nanoparticles because they are often used in actual applications. If you generate a hot carrier in such a nanosystem, it’s easier to transfer to the connected electrode due to their small sizes.’’ His calculations used light to excite Ag55, a metallic nanocluster with known geometry, and showed the behavior of the plasmon and the single particle excitation.

The study was published in a Nature Communicationspaper titled “Interplay Between Plasmon and Single-particle Excitations in a Metal Nanocluster.” Jie Ma and Zhi Wang, also from Berkeley Lab, and Lin-Wang Wang are the authors.
In the simulations, metallic nanoparticle clusters clearly responded to external light, with charge ‘sloshing’ back and forth within the clusters. However, that movement can be caused both by a plasmon and by single particle excitations. The trick is showing which is which.
‘’We found a way to distinguish them by their different oscillating behaviors. Using this method, we found that if a hot carrier excitation is in tune with the plasmon oscillation, then 90% of the plasmon energy can be converted to the single particle energy. But if they are out of tune, the total energy will go back and forth between the plasmon and the single particle exication,” explains Wang.

Jie Ma, a postdoc who is the lead author of the paper, adds that ‘’the single particle excitation is the continuous change of the electron occupation, but the plasmon is the oscillation of the electron occupations around the Fermi energy [‘ground’ level of the electron reservoir].’’ When resonance builds up between the two, most of the energy transfers to the hot carrier.
Conventional ground state computational methods cannot be used to study systems in which electrons have been excited. But using real-time simulations, an excited system can be modeled with time-dependent equations that describe the movement of electrons in the femtosecond (quadrillionth of a second) timescale.

An excited single particle can drop rapidly to a lower energy state by emitting a phonon, which is the vibration of the atoms. This means that it is no longer a hot carrier. Eventually, all hot carriers will lose their energy, as electrons and holes recombine in a metallic system. But the question is how long the hot carrier will remain hot and able to transport to another electrode or molecule before it is cooled. Previous studies, which do not include the nuclei movement, cannot describe the cooling process. But Wang’s simulation suggests that in a small nanostructure the carrier cools more slowly than in a bulk system.
‘’Here, we simulated isolated nanoparticles. But if you put the nanoparticles on some substrate, that could be really interesting,’’ says Ma. It will be important to understand how long a hot carrier can stay hot.
With powerful computational tools, these questions can now be answered and used in the development of future plasmon-driven applications.

Pines is the corresponding author of a paper in Nature Communications describing this study. The paper is titled “Room-temperature in situ nuclear spin hyperpolarization from optically pumped nitrogen vacancy centers in diamond.”
Jonathan King, a member of Pines’ research group is the lead author. Other co-authors are Keunhong Jeong, Christophoros Vassiliou, Chang Shin, Ralph Page, Claudia Avalos and Hai-Jing Wang.
The authors report the observation of a bulk nuclear spin polarization of six-percent, which is an NMR signal enhancement of approximately 170,000 times over thermal equilibrium. The signal of the hyperpolarized spins was detected in situ with a standard NMR probe without the need for sample shuttling or precise crystal orientation. The authors believe this new hyperpolarization technique should enable orders of magnitude sensitivity enhancement for NMR studies of solids and liquids under ambient conditions.
“Our results in this study represent an NMR signal enhancement equivalent to that achieved in the pioneering experiments of Lucio Frydman and coworkers at the Weizmann Institute of Science, but using microwave-induced dynamic nuclear hyperpolarization in diamonds without the need for precise control over magnetic field and crystal alignment,” Pines says. “Room-temperature hyperpolarized diamonds open the possibility of NMR/MRI polarization transfer to arbitrary samples from an inert, non-toxic and easily separated source, a long sought-after goal of contemporary NMR/MRI technologies.”
“These results are an important contribution that adds to a growing arsenal of tools being developed by experts throughout the world, including leading laboratories in the US, Europe, Japan and Israel, for creating a more sensitive NMR/MRI signature at easily attainable conditions,” says Frydman, a professor of chemistry at thes Weizmann Institute of Science. which is located in Israel, near Tel Aviv. “Achieving this could open up a plethora of applications in physics, chemistry and biology.”
The combination of chemical specificity and non-destructive nature has made NMR and MRI indispensable technologies for a broad range of fields, including chemistry, materials, biology and medicine. However, sensitivity issues have remained a persistent challenge. NMR/MRI signals are based on an intrinsic quantum property of electrons and atomic nuclei called “spin.” Electrons and nuclei can act like tiny bar magnets with a spin that is assigned a directional state of either “up” or “down.” NMR/MRI signals depend upon a majority of nuclear spins being polarized to point in one direction – the greater the polarization, the stronger the signal. Over several decades Pines and members of his research group have developed numerous ways to hyperpolarize the spins of atomic nuclei. Their focus over the past two years has been on diamond crystals and an impurity called a nitrogen-vacancy (NV) center, in which optical and spin degrees of freedom are coupled.
“An NV center is created when two adjacent carbon atoms in the lattice of a pure diamond crystal are removed from the lattice leaving two gaps, one of which is filled with a nitrogen atom, and one of which remains vacant,” Pines explains. “This leaves unbound electrons in the center between the nitrogen atom and a vacancy that give rise to unique and well-defined electron spin polarization states.”
In earlier studies, Pines and his group demonstrated that a low-strength magnetic field could be used to transfer NV center electron spin polarization to nearby carbon-13 nuclei, resulting in hyperpolarized nuclei. This spin transference process – called dynamic nuclear polarization – had been used before to enhance NMR signals, but always in the presence of high-strength magnetic fields and cryogenic temperatures. Pines and his group eliminated these requirements by placing a permanent magnet near the diamond.
“In our new study we’re using microwaves to match the energy between electrons and carbon-13 nuclei rather than a magnetic field, which removes some difficult restrictions on the strength and alignment of the magnetic field and makes our technique more easy to use,” says King. “Also, in our previous studies, we inferred the presence of nuclear polarization indirectly through optical measurements because we weren’t able to test if the bulk sample was polarized or just the nuclei that were very close to the NV centers. By eliminating the need for even a weak magnetic field, we’re now able to make direct measurements of the bulk sample with NMR.”
In their Nature Communications paper, Pines, King and the other co-authors say that hyperpolarized diamonds, which can be efficiently integrated into existing fabrication techniques to create high surface area diamond devices, should provide a general platform for polarization transfer.
“We envision highly enhanced NMR of liquids and solids using existing polarization transfer techniques, such as cross-polarization in solids and cross-relaxation in liquids, or direct dynamic nuclear polarization to outside nuclei from NV centers,” King says, noting that such transfer of polarization to solid surface and liquids had been previously demonstrated by the Pines group using laser polarized Xe-129. “Our hyperpolarization technique based on optically polarized NV centers is far more robust and efficient and should be applicable to arbitrary target molecules, including biological systems that must be maintained at near ambient conditions.”

IMAGE:
(LEFT) The research group of Alex Pines has recorded the first bulk room-temperature NMR hyperpolarization of carbon-13 nuclei in diamond in situ at arbitrary magnetic fields and crystal orientations.
(MIDDLE) (From left) Claudia Avalos, Keunhong Jeong and Jonathan King were part of a team led by Alex Pines that used microwaves to enhance NMR/MRI signal sensitivity many orders of magnitude above what is ordinarily possible with conventional NMR/MRI magnets at room temperature.