Formation Mechanisms for Phosphorene and SnIP

Abstract Phosphorene—the monolayered material of the element allotrope black phosphorus (Pblack)—and SnIP are 2D and 1D semiconductors with intriguing physical properties. Pblack and SnIP have in common that they can be synthesized via short way transport or mineralization using tin, tin(IV) iodide and amorphous red phosphorus. This top‐down approach is the most important access route to phosphorene. The two preparation routes are closely connected and differ mainly in reaction temperature and molar ratios of starting materials. Many speculative intermediates or activator side phases have been postulated especially for top‐down Pblack/phosphorene synthesis, such as Hittorf's phosphorus or Sn24P19.3I8 clathrate. The importance of phosphorus‐based 2D and 1D materials for energy conversion, storage, and catalysis inspired us to elucidate the formation mechanisms of these two compounds. Herein, we report on the reaction mechanisms of Pblack/phosphorene and SnIP from P4 and SnI2 via direct gas phase formation.

experiments and 5 gas phase species were used as input parameters. More detailed data and parameters concerning the gas phase evaluations are summarized in the Supplement Note 5.

Black phosphorus and SnIP Syntheses
Despite obvious structural differences, the synthesis procedures for both compounds are rather similar. In both cases tin, Sn-(IV)-iodide and amorphous red phosphorus (Pred) act as starting materials. In the case of Pblack catalytic amounts of Sn and SnI4 are used to form Pblack from Pred via a short way transport reaction. Bulk Pblack can be grown in a temperature range from 923 K to 823 K with a cooling rate of 13.3 K/h. [3b] In an in situ neutron diffraction experiment Pblack showed a reasonable growth rate even at lower temperatures. Within 75 minutes, Pblack grew to large crystals with edge lengths up to 0.5 cm in evacuated silica ampoules. [3b] During this experiment, a temperature window of 773 to 673 K and using a temperature cooling rate of 100 K/h. For SnIP, equimolar ratios of the elements are used instead, the reaction temperature is lowered to 673 K, and slower cooling with a gradient of 5 K/h is applied. These similarities inspired us to investigate the formation mechanism for both compounds in more detail.
SnIP crystallizes monoclinically, in space group P2/c (No. 13). It contains Z=14 formula units in the unit cell, with lattice parameters of a = 7.934 Å, b = 9.802 Å, c = 18.439 Å and  = 110.06°. [10] In 2007, Lange et al. reported on the synthesis of Pblack and identified the main gas phase species present in the reaction vessel that are P4 (I) and SnI2 (II). [16] Minor gas phase components like P2, SnI4 (III) and I2 (IV) were also taken into account, stated here in decreasing order of appearance.
No significant impurities like halide containing species were found in the final product which lead to the conclusion that no tin or iodine is incorporated in the final product. Any tin or iodine species can therefore be present only in the gas phase directing the formation of Pblack during the short way transport process. In the past few years, several studies were performed to identify a possible reaction mechanism for this Pred to Pblack gas phase transformation reaction in CVD processes. [10a] In solution a reaction mechanism has been evaluated and investigated, featuring a nucleophilic attack of a free lone pair of ethylenediamine (en) to Pwhite (P4 molecule)[7a] and Pred. [10b] Here, the P4 entity (Pwhite) is opened to a reactive species which tends to rearrange into layers of corrugated Pblack sheets afterwards. Unfortunately, this solution based synthesis with ethylenediamine leads to P-N bond formation and Nitrogen impurities in Pblack which has been proven by XPS. [10b] Such impurities which come along in addition to common oxygen impurities caused by oxidation processes in solution and during workup procedures can affect the performance of Pblack in applications. Using a gas phase-based synthesis route, the solvent influence can fully be suppressed and the oxidation problem can be minimized. Therefore, the gas phase-based synthesis remains a crucial method to grow pure, highly crystalline, large area crystals of Pblack which then can be used for a top-down fabrication of phosphorene.

In situ neutron diffraction study on black phosphorus
The formation of formation of black phosphorus was followed in situ in a Neutron diffraction experiment at the ILL, Grenoble, D20 beamline (take off angle 118°;  = 1.87 Å; cooling rate 100 K/h, PSD detector; data acquisition time 5 min. [3b] All relevant Neutron data are summarized in Figure S3 and additional experimental data or phase analysis results can be taken from literature. [3b] We found only reflections from black phosphorus in our neutron diffraction experiment and indexed the reflections according structure data [16] from the literature. The evolution of the five strongest Pblack reflections (040) Figure S3. In situ Neutron diffraction study on black phosphorus formation. Data are collected at ILL, Grenoble, D20 beamline. We observed fast growth of Pblack within approx. 100 min from 350 to 450 minutes of total experiment time. Intensities of five observed Pblack reflections, equivalent to run 72-121 in the lower section, were observed during cooling.

Note 1 -Quasi in situ observation of the synthesis processes
The videos were taken at synthesis temperature (T) directly after the oven was opened. The name of the deposited file indicates the target compound (SnIP or Pblack) in the ampoules, synthesis temperature T and the file format (.mp4). A dark red condensation product can be noticed on the walls of the Pblack ampoule while the atmosphere is orange. When opening the oven during SnIP synthesis, there is no obvious condensation process visible on the walls of the ampoule and an orange atmosphere comparable with the one in the Pblack synthesis is present. Behind the SnIP ampoule shown in the SnIP673K.mp4 file we placed three ampoules containing 25, 50 and 100 mg of iodine (4) in order to illustrate the occurrence of significant amounts of iodine in the gas phase.
In each iodine ampoule a dark violet/black gas phase is formed which is in significant contrast to the orange gas phase during the Pblack and SnIP syntheses.
Pblack923K.mp4; SnIP673K.mp4 We therefore conclude that elemental iodine is not present in significant amounts during both syntheses.

Note 2 -Gaussian data sets of DFT calculations
All data sets containing relevant structure data and additional crucial information are deposited Interested readers can access or download the data directly from the journal.

Note 3 -Barlow's Formula
Barlow's Formula is used to estimate the bursting pressure of a silica glass ampoule.
[23a] The pressure P represents the estimated maximum internal pressure for a specified tube. d and t are the diameter and wall thickness. s illustrates the tensile strengths which was chosen from the material specifications of Heraeus. [23b] The safety factor f is used to ensure a safe working environment in the laboratory.

Note 4 -Thermodynamic calculations in Gaussian
The starting point for all thermodynamic values is the partition function q for a given component.
Other variables are temperature (T), volume (V) and pressure (p), while the universal gas constant (R) is also needed.

Note 5 -Data summary
In this note we summarize data used in Figures 6 and 7.

Tragmin values
The used parameters in all TRAGMIN evaluations were extracted from the database and summarized in the data file following the program routine for data files. TragminParameters.dat