Funded Projects

Start of funding 01.07.2015

Anisotropic 2D Materials – Synthesis, characterization and optimization

Prof. Dr. Tom Nilges
Technische Universität München
Lehrstuhl für Chemie - Synthese und Charakterisierung innovativer Materialien

Prof. Dr. Chongwu Zhou
University of Southern California, Los Angeles
Electrical Engineering - Nanoloab

Two dimensional (2D) layered materials with intriguing properties have attracted significant interest in the past decades. Materials cover the full electromagnetic spectral range from semimetallic graphene [1], semiconductors like metal dichalcogenides [2] or phosphorene [3] (black phosphorus sheets), to isolating boron nitride [4].The proposed research project will deal with the synthesis of low-dimensional nano materials and devices using main group elements. We intend to prepare alloyed sheets with various compositions to fine tune the electronic properties. It is planned to fabricate transistors and sensors from this materials and to examine and optimize their electronic and optical properties. Synthesis and characterization of the raw materials will be realized in the Nilges group at Technische Universität München (TUM) and a full spectroscopic investigation of single/multilayer sheets and the fabrication of transistors will be done in the Zhou group at University of Southern California (USC).

[1] Novoselov, K. S. et al., Science 306, 666-669 (2004).
[2] Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S., Nat. Nanotechnol. 7, 699-712 (2012).
[3] Li, L. et al., Nat. Nanotechnol. 9, 372-377 (2014).
[4] Coleman, J. N. et al., Science 331, 568-571 (2011).

Final report:
The project was dealing with the synthesis, characterization and device fabrication of phosphorus-bases 2D-semiconductor devices. Target materials were orthorhombic black phosphorus (and phosphorene made of this compound by delamination), black arsenic-phosphorus and its delaminated nano-sheets and antimony-doped violet phosphorus nano rods. For all three target materials the synthesis and primary characterization was performed in Munich at TUM and samples were send to Los Angeles to the USC for device fabrication and testing.

Black phosphorus was successfully delaminated into few-layer phosphorene and used to fabricate field effect transistor devices afterwards [1]. The change of the electronic structure and response upon adsorption/desorption of NO₂ gas were used to illustrate the gas sensor capability of this 2D material phosphorene. This article is a ground-breaking milestone in the field of gas sensor science because it was shown that the sensitivity of such a sensor can be improved from the ppm to the ppb regime.

During the funding time project we were able to optimize the device properties of such phosphorene-based field effect transistors and sensors drastically. We identified a paladium-hydrogen contact as the most effective material to attach phosphorene to substrates used for field effect transistor devices [4]. Such effective contacts result in a significant improvement of the device performance and will therefore of benefit for ongoing and oncoming research in the field. This study illustrates the importance of a joint collaboration between materials scientists (at TUM) and engineers (at USC) to generate the best possible benefit for such a device optimization. Without the funding of BaCaTeC the whole black phosphorus/phosphorene project would not have been taken place.

In continuation of our 2D project we investigated new 2D materials. A substitution of 83% of phosphorus by arsenic led to black arsenic-phosphorus featuring comparable structures than pure black phosphorus. After the successful realization of bi-layers of this material for the first time, the two groups in LA and Munich were able to generate effective field effect transistors 2D devices [2]. It was obvious from this study that the significantly reduced band gap of this arsenic-phosphorus compound of 0.15 eV (bulk black phosphorus 0.33 eV) might be of interest for sensitive infrared light detection. Consequently, we fabricated few-layer IR detectors out of this material which are able to detect mid-range infrared light (wavelength 3.4 to 7.7 ?m) with a high sensitivity and very fast detection rate (11 GHz) [5].

Another project launched during the project duration was the investigation of semiconductor nano rods made of layered (or 2D) antimony-substituted violet phosphorus [3]. Delaminated to a few nanometers we build FET devices which are capable to detect visible light in a very promising and sensitive way. Upon antimony substitution the band gap was reduced from >2 eV in violet phosphorus to 1.67 eV in Sb@violett P, shifted into the visible light regime. High charge carrier mobilities and significant on/off ratios characterize this system.

The funding was used to realize a visit of Prof. Nilges from October 10th to 18th, 2015 at USC, to plan and initiate the three independent sub projects. Two students of the Nilges group (F. Baumer and D. Pfister) were send to the USC in 2016 (18.09.-29.09.2016) to perform experiments and to teach co-workers from USC to prepare and handle the materials. Knowledge of those two visits was transferred to my group and helped C. Ott to start working in this project. We intend to continue this very successful project in the future and we thank BaCaTeC for the generous support.

References of joint work in pubmed listed journals:

[1] Black Phosphorus Gas Sensors; Ahmad N. Abbas, Bilu Liu, Liang Chen, Yuqiang Ma, Sen Cong, Noppadol Aroonyadet, Marianne Köpf, Tom Nilges, Chongwu Zhou ACS Nano 9 (2015), 5618-5624. (114 citations in Google Scholar)

[2] Black Arsenic-Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties B. Liu, M. Köpf, A. A. Abbas, X. Wang, Q. Guo, Y. Jia, F. Xia, R. Weihrich, F. Bachhuber, F. Pielnhofer, H. Wang, R. Dhall, S. B. Cronin, M. Ge, X. Fang, T. Nilges, C. Zhou Adv. Mater. 27 (2015), 4423-4429. (65 citations in Google Scholar)

[3] Synthesis, Characterization, and Device Application of Antimony-Substituted Violet Phosphorus – A Layered Material Franziska Baumer, Yuqiang Ma, Chenfei Shen, Anyi Zhang, Liang Chen, Yihang Liu, Daniela Pfister, Tom Nilges, and Chongwu Zhou ACS Nano 11(4) (2017), 4105-4113.

[4] Black Phosphorus Field-Effect Transistors with Work Function Tunable Contacts Yuqiang Ma, Chenfei Shen, Anyi Zhang, Liang Chen, Yihang Liu, Jihan Chen, Qingzhou Liu, Zhen Li, Moh. R. Amer, Tom Nilges, Ahmad N. Abbas. Chongwu Zhou ACS Nano (2017), 11 (7), 7126-7133.

[5] Air-stable mid-infrared photodetectors based on pristine arsenic-phosphorus heterostructures Shaofan Yuan, Claudia Ott, Tom Nilges, Chongwu Zhou and Fengnian Xia (2017) paper submitted.