Self-organizing layers from complex molecular anions

The formation of traditional ionic materials occurs principally via joint accumulation of both anions and cations. Herein, we describe a previously unreported phenomenon by which macroscopic liquid-like thin layers with tunable self-organization properties form through accumulation of stable complex ions of one polarity on surfaces. Using a series of highly stable molecular anions we demonstrate a strong influence of the internal charge distribution of the molecular ions, which is usually shielded by counterions, on the properties of the layers. Detailed characterization reveals that the intrinsically unstable layers of anions on surfaces are stabilized by simultaneous accumulation of neutral molecules from the background environment. Different phases, self-organization mechanisms and optical properties are observed depending on the molecular properties of the deposited anions, the underlying surface and the coadsorbed neutral molecules. This demonstrates rational control of the macroscopic properties (morphology and size of the formed structures) of the newly discovered anion-based layers.


Supplementary Figures Data Page
Supplementary (opt) 33 Supplementary Tables  Supplementary Table 1: Molecular formulas Hydrocarbons  (MS)  12  Supplementary Table 2: XPS elemental composition after dewetting  (XPS)  16  Supplementary Table 3 A scheme to rationalize the retention of charge during ion soft landing is shown in Supplementary Figure 1. Note that deposition of cations on bare metal surfaces results in neutralization. In contrast, deposition on an insulating surface results in buildup of electric fields that prevent further ion deposition at high coverages. Larger amounts of ions that retain their charges can be deposited onto insulating SAMs on top of an underlying conductive surface. The landing of a cation on such a substrate induces a mirror charge in the gold that is grounded through a picoammeter. The situation can be described as loading a two-plate capacitor. If the layer of soft landed ions and mirror charges is approximated by uniformly charged round plates with radius R separated by a distance d (Supplementary Figure 1c), the force F on an ion approaching the center of the plate at distance D may be expressed using equation S1: 1 Supplementary Figure  The electric field above the center of the plates vanishes for infinitely large plates, and, therefore, becomes extremely small for large capacitors (very large R/d ratio). However, at the borders, side effects are present that result in larger electric fields that may lead to repulsion between the deposited ions of the same polarity. Therefore, such side effects tend to be minimized by forming a smooth layer instead of many "islands" of accumulated ions (i.e. formation of one large capacitor with uniform plates vs. many small capacitors in the "island model").
High coverages of cations usually resulted in discharge of the ions. This breakdown of the capacitor was explained by the buildup of an electric field across the insulating layer that is strong enough to allow electrons to pass from the gold surface through the SAM. 2 The neutralization is energetically favorable since electron attachment to cations is a spontaneous process. Recent studies showed that highly electronically stable anions stay charged after deposition (see the main text). The deposition of higher amounts of anions may be possible because excess electrons can be bound to anions by several eV. In addition, electron loss from multiply charged anions is hindered by a repulsive Coulomb barrier. This may result in the build up of larger potentials. A detailed examination of the physical situation is beyond the scope of this study, but we present data that clearly show that most of the deposited ions are not neutralized (Supplementary Figure 8 and 9).

Supplementary Note 2: Bright field image of deposition spot before dewetting
The optical appearance of a deposited spot before the dewetting process starts is shown in the following image. At this stage, the layer is invisible in the dark field. For [B12Cl12] 2dewetting usually starts too soon to map the surface. Therefore, an image of a [B12Br12] 2layer is shown, which is similar in optical appearance to [B12Cl12] 2before dewetting.

Supplementary Note 3: AFM investigation of layers in the initial state of dewetting
The initially smooth morphology of the [B12Cl12] 2based layer was demonstrated by acquiring AFM images of a freshly prepared surface directly after its exposure to air. Supplementary Figure 3 demonstrates that the layer surface is much smoother than the underlying substrate (FSAM on gold coated Si). The calculated RMS roughness is 2.30 nm for the bare FSAM surface and 0.44 nm for the layer, respectively.
To estimate the thickness of the layers, we also performed AFM measurements on layers during dewetting. Supplementary Figure 4 shows line profiles obtained over the borders of growing holes. Supplementary Figure 5 shows the development in height of these borders during the process of continuous hole expansion and merging of two borders.

Supplementary Note 4: Thickness dependency of self-organized structures
An increase in droplet size and size of the formed pattern depending on the amount of deposited [B12Cl12] 2ions was found.
Supplementary Figure 6. Comparison of the droplet arrangement after dewetting of layers formed by soft landing of different amounts of [B12Cl12] 2showing substantially larger holes at higher coverages with droplets arranged at the edges.

Supplementary Note 5: IR investigations
Layers prepared by soft-landing of [B12X12] 2ions (X=F-I) onto SAMs were analyzed using in-situ infrared-reflection-absorption spectroscopy. The assignment of IR bands to adventitious hydrocarbons that bind to the surface is supported by comparison of the spectra for different X (Supplementary Figure 10). The growth of the hydrocarbon signals was found to be correlated to the growth of the ion signal: Slower deposition (lower current) did not result in a significant change of the signal ratios. An increase in the hydrocarbon bands in proportion to the ion signals was detected during deposition, see Supplementary Figure Figure 15, in agreement with the literature. 4 We note that an additional lower intensity 11 B NMR signal (+18 ppm) was detected during our investigation of the dissolved [B12Cl12] 2layer. This chemical shift corresponds well to the 11 B chemical shift of the boric acid standard used as a reference, but species such as (RO)2BX (X=halogen) have also been observed at similar ranges of the chemical shift. The additional resonance may have formed as a decomposition product over time in these samples. The dissolved layers were stored prior to NMR investigations for 9 months. The [B12I12] 2layer was stored prior to NMR investigations only for 3 weeks and showed the same signal with more than an order of magnitude weaker relative intensity. All other analytical methods (like XPS) which have been performed in the time frame of minutes to days after deposition, did not indicate the presence of any other boron-containing molecules.

Supplementary Note 9: XRD-analysis
In an effort to obtain information on long range order, micro-XRD investigations on droplets were performed. No polycrystalline order was detected in the droplets by XRD. We did not detect any peaks that may correspond to the material in the 5-100 degrees 2-Theta range (using Cr K, radiation; 2.2897 Å). This observation further supports the liquid nature of the material.

Supplementary Note 10: Morphology of the [B12F12] 2layer and droplets
The morphology of layers generated by soft-landing of

Supplementary Note 11: Vacuum drying
We performed vacuum drying of a [B12Cl12] 2layer after the initial dewetting stage. Straight circular holes as shown in Figure 3 in the manuscript appeared in the layer prior to the process. Subsequent storage for one week in the soft landing instrument under vacuum resulted in the deliquescence of the straight hole borders showing that the driving force for the dewetting process has been removed. The hole borders did not recover, but instead new holes were formed in the areas of intact layer after some time under ambient conditions.

Supplementary Note 14: Phthalate composition
To investigate the reproducibility of the obtained results, layers were generated after the soft landing instrument was taken apart and cleaned, seals replaced, a pump repaired and the pump oil changed. Although the composition of the phthalates changed in terms of relative ratios (see Supplementary Figure 25a), the macroscopic behavior was well reproduced (Supplementary Figure 25b). The [B12Cl12] 2layers showed the previously described dewetting while [B12I12] 2layers were stable during this time frame. It was also possible to substitute the distribution of phthalates by one defined phthalate, see supplementary note 15.
Supplementary Figure 25. a) ESI, positive mode, mass spectra of a [B12Cl12] 2based layer generated after significant changes to the instrument and pumps were performed. The layer behavior is well reproduced as shown in b.

Supplementary note 15: Codeposition of a defined phthalate
By introduction of a liquid phthalate into the soft-landing instrument with base pressure 10 -8 mbar and heating of the reservoir to 80-120°C, we could substitute the distribution of phthalates almost completely with pure diisodecyl phthalate. The base pressure increased during the experiment by one to two orders of magnitude. A residual gas analyzer mass spectrum is shown in Supplementary Figure 26, which shows typical signals of diisodecylphthalates in the measurable mass range (see inserted reference spectrum from NIST Chemistry webbooks for comparison). Supplementary Figure 27 shows mass spectrometric analysis of the layer and optical microscopy of the dewetted layer.

Experiment 1: Partial substitution
For partial substitution of the adventitious hydrocarbons with glycine, we put solid glycine powder on a metal flange next to the gold surface to be used for deposition and heated it up in the vacuum chamber to 70˚C. The base pressure of 8*10 -5 Torr determined by an ion gauge did not increase measurably. IR spectra measured before ion deposition showed that glycine is not deposited in any detectable amounts via chemical vapor deposition on the surface under these conditions. However, during ion deposition, the IR spectrum changed considerably compared to experiments without glycine. Still, the IR bands of the adventitious hydrocarbons were observed, see Supplementary Figure 28c. However, new IR bands attributed to glycine were clearly present in the spectrum.

Experiment 2: Full Glycine substitution
For full substitution of glycine we performed ion soft-landing in another apparatus with a lower base pressure (1×10 -8 mbar). A glycine reservoir was heated to roughly 200˚C and the vapor was introduced into the deposition chamber via a heated leak valve. The reservoir was heated slowly in the timeframe of hours up to this temperature because only at 200˚C glycine could be detected by a residual gas analyzer attached to the main chamber (increased m/z 30 signal, the predominant species in the electron impact spectrum of glycine). This was accompanied by a pressure increase of two orders of magnitude in the deposition chamber. The experimental setup is similar to previously described instruments. 5 The

Supplementary Note 17: Electron beam induced tips analysis before and after methanol washing.
Electron beam induced tips were imaged using atomic force microscopy (AFM) in contact mode for the initial state and tapping mode for the washed state. A single tip was chosen for highresolution analysis, shown in Supplementary Figure 29. Because the tip is asymmetric, line profiles across the tip were performed in two directions, along the widest and narrowest dimensions of the post-washed tip. The position of the profile was chosen to maximize the tip height. The asymmetry of the features is not likely an AFM imaging artifact due to the relatively small aspect ratio, especially for the initial state where the asymmetry is more prominent.

Supplementary Note 18: Binding of water and diisodecylphthalate to [B12Cl12] 2-
The binding energy of a diisodecyl-phthalate molecule and a water molecule was estimated using the DFT method B3LYP/def2-tzvppd 6 including dispersion forces 7 . We note that our investigation does not include all possible conformations of the organic alkyl chains of the phthalates. The binding energies should be considered as rough estimations. The optimized geometries are shown in Supplementary Figures 30-32

Supplementary Note 19: Correlation between phthalate contact angle and final morphology on surfaces
Droplets of diisodecyl-phthalate (0.5 µL) were placed on 3 different surfaces. As can be clearly seen from Supplementary Figure 33a, phthalates have a high contact angle on FSAM, a smaller contact angle on HSAM and wet the surface to a considerable extent on the unmodified gold surface. This behavior can be clearly correlated to the final stage of the layer after long exposure to environmental conditions (see Supplementary Figure 33c). On FSAM droplets are formed, on HSAM dewetting occurs, but no comparable free surface areas are formed and on an unmodified gold surface, the layer showed no visible morphological change.
Supplementary Figure 33

Supplementary Note 20: Binding of water to [B12F12] 2and [B12I12] 2in comparison.
Supplementary Figure 34 schematically shows the disposition of a water molecule in close contact with the halogen shell of [B12F12] 2and [B12I12] 2-. The overall orientation of water molecules is governed by dipole-ion interactions between the water dipole and the negative charge of the boron clusters. This leads to the favorable attractive interaction of the water hydrogens with the negative fluorine shell in of [B12F12] 2-, which is responsible for the strong binding of water (Fig. S29a). In contrast, such favorable interaction is not possible with the slightly positive iodine atoms of [B12I12] 2-. As a result, the water molecule is bound in a staggered orientation and at a longer distance from the ion, reducing the ion dipole interaction.