Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - B-PhosphoChem (Exploration of the 2D-Chemistry of Black Phosphorous)
Teaser
Single and few layer sheets of black phosphorus (BP) represent a new class of non-carbon 2D materials. Like graphite/graphene, black phosphorus represents an allotrope of an element consisting of one atom type only. Black phosphorus has a couple of very advantageous properties...
Summary
Single and few layer sheets of black phosphorus (BP) represent a new class of non-carbon 2D materials. Like graphite/graphene, black phosphorus represents an allotrope of an element consisting of one atom type only. Black phosphorus has a couple of very advantageous properties such as a high charge carrier mobility and a small band gap. These are ideal prerequisites for exciting applications such as near- and mid-infrared electronics. In addition, many other applications such as transistors, solar cells, gas sensors, and lithium ion batteries have been proposed and predicted by theory.
On the other hand, the chemistry of BP, however, remained almost completely unexplored. This represented the state of the art at the start of the B-PhosphoChem projekt and the scientific focus of the research project is dedicated to the exploration of the chemistry, to the development and understanding of the underlying chemical and physical properties and to the characterization of the materials properties of the respective derivatives/systems.
The exploration of BP chemistry will a) allow to considerably improve the processability and increase the solubility of this nanomaterial; b) it will establish concepts for its chemical stabilization, circumventing its pronounced oxophilicity; c) it will provide the opportunity for very extended modulations and for fine tuning of the physical properties; d) it will give access to the combination of the physical and materials properties of BP with those of other compound classes; e) it will reveal at a fundamental level the intrinsic chemical properties and reactivity principles of BP; and f) it will serve as a basis for the development of practical applications of BP-based materials and hybrid architectures.
The unifying concept of B-PhosphoChem is ideally suited to accomplish these challenges because of its closely integrated and highly interdisciplinary approach consisting of five work packages:
(WP1) Production of Thin Layer BP
(WP2) Supramolecular Chemistry of BP
(WP3) Intercalation Compounds of BP (BPICs)
(WP4) Covalent Chemistry of BP
(WP5) BP-Based Materials and Devices
Work performed
As outlined in our initial time-line gant chart in the proposal, with the beginning of the B-PhosphoChem we focused our research in a highly complementary approach on the questions and objectives outlined in WP1-4. Results obtained within a specific workpackage have also influenced the progress and breakthrough in other workpackages and in the following this evolution will be summarized.
WP 1: Production of Thin Layer Black Phosphorus:
At the start of the project, the large scale and large area production of SL-BP remained unsatisfactory and was still in its infancy. Thin flakes of BP (mixtures of SL-BP, BL-BP or FL-BP) were first prepared mostly by mechanical cleavage of bulk crystals. This pioneering procedure allowed for the exploration of the fundamental physical properties of BP but is not suitable for a large-scale production. Liquid exfoliation using viscous solvents such as N cyclohexyl-2-pyrrolidone (CHP) allows for upscaling and bulk production of FL-BP. Along these lines, we have shown, that even rather stable dispersions of flakes with observable photoluminescence can be prepared. Nevertheless, this approach does not qualify for an easy generation of large area SL-BP, keeping it still an elusive goal. In B-PhosphoChem we intended to explore two approaches for the first bulk and large area production of true SL-BP, namely i) the oxidation of surface-supported reduced single-layer black phosphorus anionic sheets, generated from the Coulomb force driven exfoliation and deposition of alkali metal black phosphorus intercalation compounds and ii) CVD of volatile P sources on catalytic and templating metal and metal oxide surfaces.
Due to our staff limitations in the ramp-up phase we have focused on approach (i) and based ont the extraordinary results obtained within WP3 (details see below) we are currently in the screening phase of the suitable parameters for the liquid phase exfoliation (solvents, substrates, ultrasound/shear force) of potassium and sodium BP intercalation compounds. In preliminary experiments we have already identified benzonitrile as a very promising oxidizing agent for the oxidation of the deposited negatively charged BP sheets. The further screening of suitable oxidizing systems and their direct comparison is currently investigated.
Based on the results obtained by the non-covalent functionalization of BP (WP2) and the directly related stabilization of the single layer material against oxidation (one major drawback for the implementation of BP in functional devices/systems) we obtained a deeper understanding of the underlying oxidation dynamics and discovered a new access route towards single layer BP, not initially defined in our proposal [RSC Adv. 2019, 9,03570-3576). Here, we developed a straightforward chemical methodology for controlling the thickness of black phosphorus flakes down to the monolayer limit by layer-by-layer oxidation and thinning, using water as solubilizing agent. In detail, we have elaborated an unprecedented top-down strategy for reducing the thickness of BP flakes at will by controlling the oxidation of the flakes and removing the oxidized phosphorous species (e.g. phosphorous acid) with distilled water. As a result, we can initiate a successive layer-bilayer thinning process. The iterative thinning procedure can be terminated at will either by using non-covalent functionalization with perylene diimide molecules, which act as a protecting barrier against photooxidation or by using BMIM-BF4 ionic liquid as a passivation layer. The suitability of this thinning procedure is revealed through the preparation of field effect transistor devices, showing that the electronic properties of the flakes are not compromised. This work provides fundamental insights into a straightforward procedure for controlling the thickness of BP flakes keeping intact its electronic properties, and holds great promise in the future development of BP-based opto-electronic devices.
WP2: Supramolec
Final results
In the first 18 months of B-Phosphochem some fundamental breakthroughs and objectives, outlined in the proposal, have already been achieved at this early stage of the project. These are:
- Successfull alkali metal intercalation of black phosphorus [WP3]
- Proof of concept for the covalent functionalization of black phosphorus based on intercalated/activated BP starting materials [WP4]
Beyond that, we have also elaborated some fundamental discoveries, not proposed in the inital grant letter:
- Acess to monolayer black phosphorus by sequential wet-chemical surface oxidation [WP1]
- Catalytic activity of few-layer black phosphorus in alkylation of soft nucleophiles with esters [WP-3]
These, highly interesting and very promising discoveries will further on be followed until the end of the project.