A 40-atom gold nanocluster breaks from gold’s signature dense packing, forming square atomic planes with a central channel that accelerates electron relaxation about 80x.
(Nanowerk Spotlight) Gold refuses to behave like other metals. It ignores oxygen, shrugs off acids that dissolve platinum, and maintains its lustrous gleam across millennia. These properties made it humanity’s eternal standard of value. Yet beneath this familiar exterior lies an atomic structure that physicists considered thoroughly understood.
Bulk gold crystallizes in what scientists call a face-centered cubic arrangement, where atoms pack together with maximum efficiency. This configuration represents one of nature’s most space-efficient architectures, leaving virtually no room for voids or channels.
That efficiency comes with limitations. A material without internal pathways cannot shuttle ions or small molecules through its structure. It cannot host guest atoms in its interior or provide protected reaction sites within its core. For catalysis, energy conversion, and molecular sensing, such capabilities would prove invaluable. Bulk gold, locked into its dense cubic architecture, simply cannot offer them.
Shrink gold to the nanoscale, however, where clusters contain mere dozens of atoms, and the rules begin to bend. Researchers have coaxed nanogold into adopting unfamiliar crystalline identities: hexagonal close-packed structures resembling titanium, body-centered tetragonal phases, and exotic four-layer hexagonal variants. Each new phase brought unexpected properties, from enhanced catalytic activity for hydrogen production to novel optical behaviors.
Yet all these alternatives shared one fundamental characteristic with ordinary gold: they remained close-packed structures. The atoms still arranged themselves to minimize empty space, still denied entry to anything seeking passage through the material’s heart.
Now, a research team spanning Carnegie Mellon University, the University of Toledo, the University of Science and Technology of China, and the U.S. Department of Energy’s National Energy Technology Laboratory has broken this pattern. Published in Advanced Materials (“Phase Engineering of Nanogold: Non‐Close Packed Square Planes in A′B′ Stacking with a 0.5 Å Channel”), their work describes a 40-atom gold nanocluster with a structure unlike any previously observed: non-close-packed square atomic planes stacked in an alternating sequence, creating a 0.5 Å channel running straight through the cluster’s core. Gold, it turns out, can adopt an open architecture with a true internal pathway.
Anatomy of the X-ray crystallographic structure of Au₄₀(S-tBu)₂₄. a) Total structure, b) individual layers of the Au24 kernel, c) protection of the Au24 kernel by dimeric staple-like motifs (only two are shown for clarity, 8 total), and d) the superlattice structure. (click on image to enlarge)
The nanocluster, designated Au₄₀(S-tBu)₂₄, consists of 40 gold atoms protected by 24 tert-butyl thiolate ligands, sulfur-containing molecules that cap the cluster’s surface. The researchers synthesized it through careful reduction of a gold-thiol precursor using mild reducing agents, then isolated the product through thin-layer chromatography.
X-ray crystallography exposed the atomic details. The gold atoms form a 24-atom inner kernel arranged as four distinct layers of square planes. Unlike conventional gold phases, where atoms tessellate into close-packed triangular arrays, these planes present open square geometries stacking in a staggered configuration. The centers of all four layers align perfectly, producing a tunnel running through the entire nanocluster.
This tunnel distinguishes Au₄₀(S-tBu)₂₄ from essentially all previously reported nanoclusters, which the researchers describe as closed structures without open channels connecting interior and exterior environments. The new structure represents a genuinely non-compact phase in nanogold.
The existence of this phase becomes particularly interesting when compared to a structurally similar counterpart. A previously reported nanocluster, Au₄₀(o-MBT)₂₄, shares the same gold-to-ligand ratio but uses a different ligand called ortho-methylbenzenethiolate. Despite this seemingly minor chemical difference, that cluster adopts a conventional face-centered cubic structure with no internal channel. The bulkier tert-butyl groups in the new cluster create less dense packing on the surface, and this altered surface chemistry apparently stabilizes the radically different core geometry.
The structural contrast produces dramatic differences in electronic behavior. Using ultrafast laser spectroscopy, the team measured how long excited electrons persist in each structure after absorbing light. The non-compact Au₄₀(S-tBu)₂₄ shows an excited-state lifetime of just 7.7 ns. The face-centered cubic version maintains its excited state for approximately 640 ns, over 80 times longer.
This acceleration in electron relaxation stems from stronger coupling between electronic and vibrational motions in the non-compact structure. The researchers identified a structural feature that may explain this effect: eight tetrahedral units of four gold atoms each form square arrays at the cluster’s top and bottom. A similar arrangement appears in hexagonal close-packed Au₃₀ nanoclusters, which exhibit comparably short 1 ns lifetimes. The cyclic arrangement of these tetrahedral building blocks appears to enable rapid energy transfer from electronic excitations to atomic vibrations.
Optical measurements show the cluster absorbs light starting around 800 nm wavelength, corresponding to an optical gap of 1.55 eV. The cluster emits weak near-infrared light centered at 850 nm with a quantum yield of just 1%, consistent with strong non-radiative decay pathways. Theoretical calculations reproduced the experimental absorption spectrum, confirming the structural assignment.
The potential applications span multiple fields. Short excited-state lifetimes coupled with strong electron-vibration coupling could prove valuable for terahertz emission, the generation of electromagnetic radiation at frequencies between microwave and infrared.
The 0.5 Å channel, while too narrow for most molecules, could permit passage of protons and other small ions. This opens possibilities for selective ion channeling relevant to artificial photosynthesis and biocatalysis, where controlled proton transport plays crucial roles. New host-guest chemistry exploiting the channel represents another avenue for exploration.
The ability to create both long-lived and short-lived excited states through phase control holds practical significance. Long lifetimes benefit photocatalysis, where sustained electronic excitation drives chemical reactions. Short lifetimes suit applications like single-photon emission and biomedical imaging, where rapid cycling matters. That ligand selection alone can toggle between these regimes by restructuring the atomic core provides a powerful design tool for nanomaterial engineers.
This research demonstrates that gold’s crystalline behavior at the nanoscale extends beyond variations on close-packed themes. The creation of a deliberately open structure establishes a new category of nanogold architecture, one where internal channels might eventually transport ions, host reactive guests, or enable chemistry impossible in gold’s conventional dense phases.
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