Cold Welding of Au Nanostructures at Room Temperature

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1.College of Science，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China；2.Department of Mathematics and Physics，Nanjing Institute of Technology，Nanjing 211167，P.R.China；3.Department of Mathematics and Physics，Jinling Institute of Technology，Nanjing 211169，P.R.China

Abstract: The common Au nanostructures（nanospheres，nanorods and nanosheets）were prepared by the seed growth method to explore the cold welding phenomenon of these non-single crystal nanostructures at room temperature. Systematic studies show that the concentration of surfactant cetyltrimethylammonium bromide（CTAB）and drying conditions are important factors to determine the evolution and final configuration of nanostructures during welding. The key factor of cold welding is the concentration of surfactant as low as 0.3 mm / L，and the welding should be carried out under the condition of slow evaporation and sufficient relaxation time，rather than rapid drying process. At the same time，the structural evolution during the welding process of gold rod head and tail is simulated by combining the electronic microscope characterization and density functional theory，which reveals that the stability of the welding nanostructure is better than that of the dispersed nanostructure.In the slow evaporation process of Au nanostructures with the same crystal structure，the low surfactant attached to the surface of the nanoparticles increases the attraction between the nanoparticles，which makes the nanoparticles close to each other adhere due to the interaction，and improves the physical properties of the intersection due to the diffusion，epitaxy and surface relaxation of the metal surface atoms. The results provide a research basis for the physical property analysis of nanostructures and the construction of defect devices.

Key words：cold welding；Au nanostructures；oriented-attachment；room temperature；density function theory

0 Introduction

In the past few years，extensive efforts have been devoted from self-assembly of metal nanostruc?tures to multifunctional units due to the particular coupling effects of surface plasmon resonance（SPR）［1］. The coupling effect arises from the inter?action between small-sized nanoparticles，which is sensitive to the spatial arrangement of nanoparti?cles，the interparticle distance and the particle num?ber of assembly. Particularly，polymers and small organic ligands are often used as stabilizers to achieve the desired self-assembling structures［2-3］，which prevents the aggregation fo the adjacent parti?cles that adsorbed on the surface of the nanoparti?cles. It is worth to note that gold nanoparticles are easy to approach and coalescence under an electron beam，similar to the welding process［4-5］，which has been developed as a fabrication technique involving the process from the fusion and cooling of metals to the construction of a strong joint，such as ion beam deposition［6-7］，thermal or laser heating［8-9］，ultrason?ic irradiating［10］，and high-energy electron beam bombardment［11-12］.However，the process usually re?quires critical temperature and reaction condition，thus it is difficult to control localized heating at the nanoscale. Cold welding of nanostructures（nanopar?ticles［13］，nanorods［4］，and nanowires［14-16］）at room temperature have been investigated since two gold thin films were joined together in the 1940s. The metal nanostructures with clean surface can rapidly form a neck with the adjacent particles even at small thermal activation，leading to surfactant-stabilized gold nanoparticles［17-18］or nanorods［19-20］assembled to micrometric-length gold nanowires by selective self-organization and cold welding［21-25］. Cold weld?ing at the nanoscale can be widely applied into opto?electronic devices due to its operability in ambient condition and spontaneous nature. In addition，the coupling effect，especially for the localized field en?hancement induced by the interaction of SPR would be extinct，as confirmed by finite-difference time-do?main simulation. The characteristic modes（such as dipole or multipole resonance）are dominated in the resonance modes of the product［26-28］. While，current research mainly focuses on the process from cold welding of ultrathin single-crystalline nanostructures to the formation of nanoaggregations. The welding mechanism of the twinned crystalline nanostructures need to be further demonstrated.

Herein，we study cold welding phenomena of twinned crystalline nanostructures including Au nanoparticles，nanorods and nanoplates at room temperature. Effects of the structural configura?tions，surfactant concentration and drying condition on the welding are explored. The surfactant concen?tration is an important factor，which directly deter?mines the cold welding of nanostructures. Combined with the experimental observation and density func?tion theory（DFT）simulation，the structural evolu?tion during the welding process was studied. It also can be found that the stability of the welded nano?structures is better than that of separated nanostruc?tures due to the rearrange of atoms for reducing the surface energy. Cold welding has been regarded as a promising nanofabrication technique in ambient con?dition.

1 Experiment

1.1 Materials

Sodium tetrahydridoborate（NaBH4，99%），hydrochloric acid（HCl，37%），Tetrachloroaurate（HAuCl4·4H2O， 99.9%） and ascorbic acid（AA，≥99.7%） were obtained from Sinopharm Chemicals. Cetyltrimethylammonium bromide（CTAB，99%）was obtained from Nanjing Robiot Co. Trisodium citrate（Na3C6H5O7·2H2O）was ob?tained from Chengdu Kelong Chemicals. Deionized water（18.25 MΩ）was used in the experiment. All reagents were used without further purification.

1.2 Preparation of Au nanostructures

Au nanostructures （nanoparticles， nanorods and nanoplates）were synthesized with a rapid seedmediated method. Briefly，the seed solution was prepared by addition of ice-cold，freshly prepared NaBH4（0.15 mL，0.01 mol/L）to an aqueous solu?tion composed of trisodium citrate （0.25 mL，0.01 mol/L）and HAuCl4（0.125 mL，0.01 mol/L）. Then solution was stirred in the oil bath at 45 ℃for 15 min and aged for 2 h at room temperature for further preparation of Au nanostructures.

（1）Preparation of Au nanorods. An aqueous solution of CTAB（10 mL，0.1 mol/L），HAuCl4（50 μL，0.05 mol/L），and 50 μL of freshly pre?pared ascorbic acid（AA，0.1 mol/L）solution was used as the growth solution.The growth solution be?came colorless when the freshly prepared AA（50 μL，0.1 mol/L）solution was added. The color?less solution was added to the as-prepared seed solu?tion（50 μL）stepwise in intervals of 30 s. The reac?tion system was kept at 20 ℃for 30 min. And the twinned Au nanorods colloids were obtained. The preparation of twinned Au nanorods is different from that of single crystalline Au nanorods，in which Ag+were applied and pH was adjusted.

（2） Preparation of Au nanopaltes. The Au nanoplates were prepared through adding foreign io?dide ions in the growth solution and modifying the experimental parameters. An aqueous solution com?posed of CTAB（10 mL，0.1 mol/L），HAuCl4（50 μL，0.05 mol/L），KI（45 μL，0.01 mol/L），NaOH（50 μL，0.1 mol/L）and freshly prepared AA（0.2 mL，0.1 mol/L）was used as the growth solution. The growth solution became colorless when the freshly prepared AA solution was added.The colorless solution was added to the as-prepared seed solution stepwise in intervals of 30 s. The reac?tion system was kept at 30 ℃for 20 min for the growth of Au nanoplates.

（3）Preparation of Au nanospheres. Au nano?spheres will be dominated in the sample，when the concentration of the reactants in the growth solution are deviated from the equilibrium value of the above nanorods and nanoplates.

1.3 Cold welding

The prepared samples （Au nanoparticles，nanorods and nanoplates）were dropped on a carboncoated copper grid. And the cold welding of twinned nanostructures were carried out by a natural slowevaporation process distinguished from the normal fast-dry on the substrate at room temperature（25—30 ℃），which is very similar to the general treat?ment method. The main difference is that the cold welding occurs without necessary for a fast-drying evaporation，and directly derives from the formation mechanism of the natural slow-evaporation process.

The overall morphologies of the samples were observed by transmission electron microscopy（TEM）JEOL-100CX and high-resolution transmis?sion electron microscope（HRTEM）JEOL—2011.

2 Results and Discussion

Various of Au nanostructures， including nanoparticles，nanorods and nanoplates，were fabri?cated by seed-mediated method. Fig.1 shows the structural TEM images and corresponding HRTEM images（inset）. It can be indicated that the nanopar?ticles（Fig.1（a））and nanorods（Fig.1（b））are fivetwinned structures［29-30］，and nanoplates is a crystal?line structure with（111）basal plane（Fig.1（c）），accompanied with some single-twined crystalline in?duced from stacking-fault seeds［31］.

Fig.1 TEM images and the corresponding HRTEM imag?es (inset)

Interestingly，the nanochain （or pair） com?posed of nanoparticles，nanorods，and nanoplate could be induced when the TEM samples were pre?pared by a natural slow-evaporation process differ?ent from the classical fast-evaporation method.Fig.2 show the TEM and HRTEM images of Au nanostructures under dry in the air. The assembled nanostructures are oriented-attachment with sur?rounding particles，and are welded together. Obvi?ously，the nanoparticles with diameter of about 30 nm are analogous to nano-decahedron. According to the reported quantitative equilibrium phase map［32］，it can be known that the decahedron over 15 nm is not a stable nanostructure. Therefore，large size of nano-decahedron would undergo Os?wald ripening，to transfer into more stable nano?structures. For the touched Au decahedrons，outer atoms would move toward the neck，resulting in the formation of nanochain.

Fig.2 TEM images and the corresponding HRTEM imag?es of Au nanochains composed of nanoparticles and nanorods

In order to further investigate the formation of welding，anisotropic Au nanostructures were pro?cessed with the similar method. From the TEM im?ages of Fig.3（a）and Fig.3（b），it can be observed that Au nanorods are stable after three months at room temperature. However，when the concentra?tion of surfactant CTAB is decreased，the welded Au nanorods can be formed. The critical concentra?tion of surfactant bilayer CTAB is less than 0.3 mmol/L. When the concentration of CTAB is more than 0.5 mmol/L，no welding evolution dur?ing the assemble process can be observed，only sev?eral of configurations（such as end-to-end and Lshaped welding）can be formed，as shown in Fig.4（a），which is different from the conventional weld?ing，since no mechanical force and external energy are exerted on the nanostructures surface. It is dem?onstrated by the formation of nanoparticle chains in the presence of CTAB（0.25 mmol），as shown in Fig.4（b）. Additionally，nanochains or nanopairs composed of Au nanorods with different aspect ra?tios can be formed at low concentration of surfactant（0.25 mmol/L），as shown in Figs.5（a—c），illus?trating the weld phenomenon is independent on the size of Au nanorods within the nanometer size range.

Fig.3 TEM images of Au nanorods colloids

Fig.4 TEM images of Au nanostructures with differ?ent concentration of CTAB

Fig.6 shows the junctional details between dif?ferent Au nanoparticles. It can be found that these nanostructural junctions present different crystalline planes due to Au atoms redistribute along with the interface between nanoparticles. Different types of welding including end-to-end（Fig.6（a1））or Lshape（Fig.6（b1））for nanorods with intertwinning crystalline planes can be captured. Additionally，it can be observed that nanorods can be combined and welded with nanoparticles（Fig.6（c1））and one-di?mensional nanoplates（Fig.6（d1））. This illustrates that the evolution and final configuration of nano?structures depend on the initial structure and state of the system，and cold welding is irrelevant to the ini?tiation of the crystal indices.

Fig.5 TEM images of Au nanorods with differ?ent aspect ratios in the presence of CTAB(0.25 mmol/L)

Fig.6 HRTEM images of welding junctions for differ?ent types of nanostructures

Furthermore， from the corresponding HR?TEM images in Figs.6（a2，b2，c2，d2），it can be demonstrated that all dislocations at this observed grain boundary have the shortest Burgers vector，which indicate that the welded nanochain experienc?es a stretching process，where a plastic deformation can be formed. Plastic deformation proceeds gradu?ally with the emission of dislocation and annihilation of boundary，indicating that structural reorganiza?tion occurred at the cross-butting point of the nanoparticles，which can be attributed to the orienta?tion and surface relaxation of the atoms in the nanoparticle.

In addition to the concentration of surfactant，the effect of evaporation condition on welding was also investigated. The nanoparticles colloids in the presence of CTAB（0.25 mmol/L）were dropped on the TEM substrates，following dried at room temperature（25—30 ℃）and heated oven，respec?tively. It can be found that the samples dried in the heated oven would quickly deposit and mono-dis?persed on the substrate，as shown in Fig.7（a）.While the nanoparticles would interact with adjacent particles during the process of natural slow-evapora?tion，as shown in Fig.7（b）. Obviously，high tem?perature can promote the drying and depositing of the samples on the substrate，decreasing the interac?tion of the adjacent particles during the process of natural drying. Fig.7（c）shows the schematic illus?tration of different drying processes. The low con?centration of surfactant and interaction of nanoparti?cles lead to the cold welding by particles attach?ment. The oriented-attachment is attributed to the strong Van der Waals force. In addition，the colloi?dal surfactant must be sufficiently weak to allow the nanoparticles to approach each other down a critical distance，where Van der Waals interactions within this distance can cause further attraction［19］.

Fig.7 TEM images of dried Au nanostructures

The diffusion barrier for a single metal atom on a clean surface of nanostructure is about 1.0 eV，which is enough for small thermal activation to trig?ger atomic diffusion［13，33］. Due to the high surface en?ergy on the end surface of nanorods，the slightly oblique with a few internal defects of nanorods is formed before welding. A few dislocations nucleate and propagate on the（111）close packed plane（slip plane）on the surface of the nanorods. Thus，the two approaching nanorods are initially welded with an incomplete jointing area，and emerged to a single nanorod under the Van der Waals attractive force.

Based on the experimental results，DFT was also applied to explore the evolution of Au nano?structures and corresponding strain energy for the welding of head-to-head nanorods. The caculated structural evolution is consistent with the experimen?tal observation，as shown in Fig.8. In the simula?tion，the initial distance between the two nanorods is set as 9 nm，and then the two nanorods approach slowly at a certain step size（450—650 steps with 0.02 nm/step）with a mechanical force. Fig.8（a）shows the structural evolution from the two ap?proaching nanorods to the welded nanostructure un?der the mechanical force. An obvious atoms diffu?sion，oriented attachment and surface relaxation can be observed，as well as lattice recombination at the cross-point can be captured. This phenomenon is consistent with the experimental results shown in Fig.8（b）. Meanwhile，the strain energy as a func?tion of displacement in the cold welding of head-tohead nanorods is shown in Fig.8（c）. In which，the abscissa describes the relative distance between the nanorods and initial position. Compared with mono?mer，the strain energy of welded nanostructures is decreased，illustrating that the stability of welded nanorods is improved，which can be attributed to the minimized surface caused by rearrange of the at?oms as the external force is transformed to the strain energy of the structure. Small-sized crystal with higher surface-to-volume ratio possesses much more high-energy adatoms，which tend to escape from the active sites to form thermodynamic stable crys?tal.

Fig.8 Structural evolution and property analysis for welding of end?to?end nanorods

3 Conclusions

In summary，various of Au nanostructures（Au nanoparticles，nanorods and nanoplates）were successfully synthesized，and cold welding phenom?ena of these non-single crystalline nanostructures without crystals melting were explored. Systematic studies reveal that the surfactant（CTAB）concen?tration and drying condition were important factors which determined the evolution and final configura?tion of nanostructures on the welding. The concen?tration of surfactant as low as 0.3 mmol/L is the crit?ical element of cold welding，and welding occurs at slow evaporation with necessary for enough relax?ation time as opposed to in-situation deposition for rapid drying. Combined with the experimental obser?vations and DFT simulations，the structural evolu?tion in the welding process is studied. It also can be found that the stability of the welded nanostructures is better than that of dispersed nanostructures due to the improved physical properties in the cross-point caused by the diffusion，epitaxy and surface relax?ation of atoms. Cold welding may provide great op?portunities in low-cost nanofabrication of nanode?vice.