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This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au_{20} cluster can be modulated by incorporating Ag, and the HOMO–LUMO gap of Au_{20−}_{n}_{n}

Gold nanostructures have attracted considerable attention owing to their unique electronic and optical properties, as well as their great potential for medical applications [

Small gold clusters of typically 1–2 nm are proposed as alternative materials. Sun _{2} clusters can induce the unusual optical transition, and the optical absorption can be modulated to the NIR. This is also confirmed by the recent photothermal therapy and drug delivery experiment [_{4}–Au_{13}, while the Au_{14}–Au_{20} show the cage-like three dimension structure [_{32}–Au_{38} clusters have been predicted, although the experimental result may always be contradicted. The optical absorption of Au_{2}–Au_{13}, Au_{19}, and Au_{20} have been calculated by time-dependent density functional theory (TDDFT), while the optical transition of Au_{32} is also focused due to its more stable structure. Notably, the tetrahedral Au_{20} cluster shows a band gap of ∼1.818 eV, and shows slight NIR absorption [

Doping Au clusters by other metals provides an available route to modulate electronic and optical properties [_{M}_{N}

Here, we studied electronic structure and optical properties of Ag-doped Au_{20} clusters. The paper is organized as follows. Section 2 presents and discusses the results of our calculations. First, we investigated the structural properties by analyzing the binding energy. Then we calculated the electronic structures, because the optical properties depend on both the interband and intraband transitions, which are determined by electronic states. Finally, we analyzed the optical transition in different configurations. Section 3 describes the basic ingredients and details of computational methods we applied. Section 4 concludes and summarizes our findings.

_{20}_{−n}_{n}_{b}_{n}_{20} is 2.40 eV, which is very close to the previous investigation of gold clusters [_{19}Ag_{1}, Au_{18}Ag_{2}, Au_{17}Ag_{3}, and Au_{16}Ag_{4} are 2.68, 2.68, 2.68, and 2.69 eV, respectively. The increasing doping atom induces a tiny effect on the binding energy. It is worth noting that the binding energy of AuAg alloy is higher than the Au_{20}. It shows that Ag atom incorporation can enhance the structural stability. Indeed, the Au–Ag bond is stronger than the Au–Au bond and gives an extra σ-bonding interaction by the overlap between the vacant Ag 4

_{20}_{−n}_{n}_{20} cluster shows the large HOMO-LUMO gap, which is in good agreement with the other computational results [_{20} is 1.47 eV, which is less than the experimental data of 1.78 eV (or 1.818 eV) due to the underestimation of electronic states by DFT [_{20}_{−n}_{n}_{20} cluster expect for Au_{16}Ag_{4}. It confirms that the Ag incorporation into Au_{20} can induce the obvious effect on gap, which is consistent with the previous results [_{20}_{−n}_{n}_{19}Ag_{1}, Au_{18}Ag_{2}, and Au_{17}Ag_{3} is shifted to the low energy range compared with Au_{20}, which can be clearly seen in _{16}Ag_{4} shift to high energy range, and thus induce the increase of the gap, which should be related to improving structural stability and enclosing electronic configurations [

For investigating the optical transition of Au_{20}_{−n}_{n}_{20} in _{2}(_{1}, _{2}), which are very close to the previous results of 1.86 and 2.78 [_{1} (1.79 eV) should mainly be caused by optical transitions between Au 6_{20}, the Au _{2} can be due to the optical transitions between HOMO-1 consist of Au

_{2}(_{20}_{−n}_{n}_{1} has gradually disappeared, which can be related to the red-shift of _{2} and further inhibition of the intrinsic optical transition of _{1}. Secondly, _{2} shows the tunable optical properties with the increasing Ag incorporation. The _{2} of Au_{20}, Au_{19}Ag_{1}, Au_{18}Ag_{2}, Au_{17}Ag_{3}, and Au_{16}Ag_{4} is 2.51, 2.35, 2.25, 2.07, and 2.01 eV, respectively. To understand these optical phenomena in detail, it is necessary to analyze the optical transition by electronic states.

_{20}_{−n}_{n}_{19}Ag_{1} and Au_{18}Ag_{2}, Ag electronic states contribute to both HOMO and LUMO. Furthermore, LUMO has slightly shifted to the low energy range, which induces the decrease of transition level and can be responsible for the red-shifts of E_{2}. In the second stage of Au_{17}Ag_{3} and Au_{16}Ag_{4}, the increasing Ag atom induces more dispersive in _{16}Ag_{4} is important evidence for the enhancement of binding energy and structural stability [

_{20}_{−n}_{n}_{20}_{−n}_{n}_{20}, the absorption band of 450 nm could be due to the optical transition of _{2}, while the 707 nm absorption band is related to the intrinsic optical transition of _{1}. The Ag incorporation induces the red-shift of absorption band (_{2}) from 478 nm (Au_{19}Ag_{1}) to 543 nm (Au_{16}Ag_{4}). We need to consider two possible effects on the optical absorption of these Au_{20}_{−n}_{n}_{20} from the experiment, the gap of tetrahedral Au_{20} is about 1.7–1.8 eV, which is larger 0.23–0.33 eV than the calculated gap of 1.47 eV. Thus, the actual gap of Au_{20}_{−n}_{n}_{20}_{−n}_{n}_{16}Ag_{4} is higher than that of the pure Au_{20} cluster and the other AuAg clusters, which indicates the doping feasibility. Therefore, more Ag atom incorporation may be promising for further fabrication, NIR absorption and related applications. Zorriasatein _{20} clusters. These methods also have potential applications in understanding the optical properties of metal nanoclusters and designing materials for photothermal therapy.

The calculations are based on density functional theory (DFT) using a plane-wave pseudopotential method [_{20}_{−n}_{n}^{10}6^{1} outermost valence electrons of the Au atom and 3^{10}4^{2} outermost valence electrons of the Ag atom are described. It is well known that the interaction of a photon with the electrons in the system can be described in terms of time-dependent perturbations of the ground-state electronic states. Optical transitions between occupied and unoccupied states are caused by the electric field of the photon. The spectra from the excited states can be described as a joint density of states between the valence and conduction band. The momentum matrix elements, which are used to calculate the _{2}(_{1}(_{2}(

The calculations are performed at 20 × 20 Å supercell, which contains 20 neutral Au atoms. These models of gold clusters refer to the previous work, which has shown that the best structural stability and average distance between Au-Au bonding is about 2.73 Å [_{20}_{−n}_{n}_{20}_{−n}_{n}^{−6} eV/atom in the self-consistent calculation. And then, the Au_{20−}_{n}_{n}^{−5} eV/atom, 0.05 eV/Å, 0.1 GPa, and 0.002 Å, respectively.

In summary, a first-principles study has been performed to evaluate the electronic and optical properties of Au_{20}_{−n}_{n}_{20}_{−n}_{n}_{20} cluster. The increasing Ag concentration can induce the HOMO-LUMO gap variation. Subsequently, the optical transition between HOMO-LUMO has shifted to the low energy range with the increasing Ag concentration. Tunable optical transition has been observed, and was shown to decrease from 2.51 to 2.01 eV with the increase of Ag atoms. Our results clearly show that Ag incorporation can modulate the optical properties of Au_{20} clusters.

This work is supported by the National Natural Science Foundation of China (Grant No. 81000668), the Specialized Research Fund for Doctoral Program (SRFDP) of Higher Education State Education Ministry (Grant No. 200800231058), and the Subject Development Foundation of Institute of Radiation Medicine, CAMS (Grant No. SF1003).

_{2}Cluster: A Nanobullet for Tumors

_{7}, Au

_{19}, and Au

_{20}clusters in the gas phase

_{32}cluster with fullerene symmetry

_{20}: A tetrahedral cluster

_{20}cluster: a TDDFT study

_{n}

_{2}(

_{20−x}Cu

_{x}

_{19}X Clusters (X = Li, Na, K, Rb, Cs, Cu, and Ag)

_{n}

_{3}Au

_{10}

Calculated ground state geometries of Au_{20}_{−n}_{n}

The partial DOS of (a) Au_{20}_{−n}_{n}

The imaginary part of dielectric function _{2}(_{20} clusters.

The tunable imaginary part of dielectric function _{2}(_{20}_{−n}_{n}

The outline of optical transition of Au_{20}_{−n}_{n}

Optical absorption of Au_{20}_{−n}_{n}