Golden nanoglue completes the wonder material – X-rays prove it

Modern microelectronics relies on semiconductors and their metal electrodes. High-performance device functionality demands high transistor density within a single chip, which soon will reach the physical limits of bulk materials. Alternatives have been found in atomically thin materials, e.g. graphene and its semiconductive inorganic relatives.

MoS2 (molybdenum disulphide) is the representative inorganic layered crystal with properties similar to those of graphene. To be useful in applications, it must be joined to the metallic electrodes to enable charge flow between the metals and semiconductive (M/S) counterparts. In a recent study, scientists from University of Oulu, Finland have demonstrated the success of joining MoS2 to Ni (nickel) particles by using gold (Au) nanoglue as a buffer material. Through in-house observations and the first-principles calculations, the semiconductor and metal can be bridged either by the crystallized gold nanoparticles, or by the newly formed MoS2-Au-Ni ternary alloy.
A metallic contact is formed, leading to enhanced electron mobility crossing the M/S interface.

MoS2-Au-Ni ternary alloy. Nickel in blue, gold in gold, MoS2 in black and green. The enhanced electron mobility is visualised in yellow lines.

Despite the success in electronic engineering, evidence of M/S interface at the electronic structure level is hardly reached through conventional methods. A simultaneous microscopic and spectroscopic determination is desired to figure out the physical signature at nanometre scales. Experiments performed at the PEEM endstation of the beamline I311* at MAX IV Laboratory take a critical step forwards in such an exploration. As for nickel, the X-ray absorption recorded at the M/S interface showed an additional peak compared with the metallic particles. This signature proves the Ni alloying with the MoS2 parts with the participation of Au, as given by the computational results. Further analysis of the peak intensity can serve as a unique path to reveal the effective number of metal atoms involved in the M/S interface. Top layers of the Ni particles are identified as the interaction channels with the layered counterpart.

More details in the Inside Front Cover article at the journal Small and an article in Microscopy and Microanalysis.

Metallic Contact between MoS2 and Ni via Au Nanoglue. Xinying Shi, Sergei Posysaev, Marko Huttula, Vladimir Pankratov, Joanna Hoszowska, Jean‐Claude Dousse, Faisal Zeeshan, Yuran Niu, Alexei Zakharov, Taohai Li, Olga Miroshnichenko, Meng Zhang, Xiao Wang, Zhongjia Huang, Sami Saukko, Diego López González, Sebastiaan van Dijken, Matti Alatalo, Wei Cao. Small, Volume 14 (2018), Issue 22, article 1704526.

Quantification of Bonded Ni Atoms for Ni-MoS2 Metallic Contact through X-ray Photoemission Electron Microscopy. Xinying Shi, Marko Huttula, Vladimir Pankratov, Joanna Hoszowska, Jean-Claude Dousse, Faisal Zeeshan, Yuran Niu, Alexei Zakharov, Zhongjia Huang, Gang Wang, Sergei Posysaev, Olga Miroshnichenko, Matti Alatalo, Wei Cao. Microscopy and Microanalysis, Volume 24 (2018), Issue S2, 458-459.

Oulu University Team (from left to right): Doc. Wei Cao, Prof. Marko Huttula, Sergei Posysaev, Prof. Matti Alatalo, Sami Saukko, Dr. Xinying Shi. Missing in the photo: Dr. Vladimir Pankratov and Olga Miroshnichenko.

*The results published in Small, Volume 14 (2018), Issue 22, article 1704526 are based on experiments performed at the beamline I311 on the old MAX II storage ring. The endstation, photoelectron microscope, is now transferred to the new 1.5 GeV ring and the new dedicated beamline MAXPEEM which has much better performance than I311, with a photon flux 100 times higher, that enables researchers performing experiments which were unthinkable at the old facility (decommissioned December 2015).