Laser ablation of molecular carbon nitride compounds

Applied Surface Science 349 (2015) 353–360

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Laser ablation of molecular carbon nitride compounds D. Fischer a,∗ , K. Schwinghammer a,b,c , C. Sondermann a,b , V.W. Lau a , J. Mannhart a , B.V. Lotsch a,b,c a b c

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany Department of Chemistry, University of Munich, LMU, Butenandtstr. 5-13, 81377 Munich, Germany Nanosystems Initiative Munich (NIM) and Center for Nanoscience (CeNS), 80799 Munich, Germany

a r t i c l e

i n f o

Article history: Received 21 January 2015 Received in revised form 20 April 2015 Accepted 28 April 2015 Available online 7 May 2015 Keywords: Pulsed laser deposition Ultra-short laser Carbon nitrides Thin films Properties

a b s t r a c t We present a method for the preparation of thin films on sapphire substrates of the carbon nitride precursors dicyandiamide (C2 N4 H4 ), melamine (C3 N6 H6 ), and melem (C6 N10 H6 ), using the femtosecondpulsed laser deposition technique (femto-PLD) at different temperatures. The depositions were carried out under high vacuum with a femtosecond-pulsed laser. The focused laser beam is scanned on the surface of a rotating target consisting of the pelletized compounds. The resulting polycrystalline, opaque films were characterized by X-ray powder diffraction, infrared, Raman, and X-ray photoelectron spectroscopy, photoluminescence, SEM, and MALDI-TOF mass spectrometry measurements. The crystal structures and optical/spectroscopic results of the obtained rough films largely match those of the bulk materials. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The remarkable mechanical, optoelectronic and structural properties of polymeric carbon nitrides have been attractive targets for both fundamental and applied research since the 19th century. Following the “harder than diamond” postulate from Liu and Cohen in 1989 for cubic carbon nitride [1], many synthetic approaches have been explored to yield this hypothetical structure, including physical methods (e.g., atmospheric-pressure chemical processes, ion-beam deposition, laser techniques, chemical vapor deposition, and reactive sputtering) [2–4] and chemical routes (e.g., thermal condensation of molecular precursors) [5–10]. Whereas physical methods resulted in mostly amorphous carbon nitrides with insufficient nitrogen content, chemical routes seemed to be more successful. Still most of the thermal condensation reactions yielded amorphous or hydrogen/oxygen-containing products, such as either melon ([C6 N7 (NH2 )(NH)]n ) [11], poly(heptazine imide) [12], or poly(triazine imide) [13,14]. Recently, Bojdys and coworkers presented a crystalline triazine-based graphitic carbon nitride with very low hydrogen content (0.51 wt%) and a carbon-tonitrogen ratio close to the theoretical one for g-C3 N4 [15]. Owing to their optoelectronic properties, these polymers have nonetheless

∗ Corresponding author. Tel.: +49 7116891532. E-mail address: d.fi[email protected] (D. Fischer). http://dx.doi.org/10.1016/j.apsusc.2015.04.212 0169-4332/© 2015 Elsevier B.V. All rights reserved.

found potential applications, from light-induced catalysis including hydrogen evolution from water [16,17] to chemical sensing [18,19] and electronic devices [15,20]. Advancement in these applications hinges on the detailed characterization of their physical properties, which so far have been limited due to difficulties in material processing. Our interest is to controllably obtain thin films of carbon nitrides and their precursors on various substrates, which would enable their characterization with surface sensitive techniques. Here, we explore a variant of the pulsed laser deposition (PLD) to grow precursors of carbon nitride (CNx ) materials on flat substrates. These films can be subsequently treated (e.g. by thermal treatment) to form novel CN networks, which may provide further insights into the condensation pathways of different carbon nitride precursors, which are not well understood yet. While PLD is a method to stoichiometrically transfer materials from a target to any substrate [21–23], it is limited to compounds that are stable under laser irradiation, thus hampering the success of organic compounds deposited by PLD [24]. Generally, CNx films were formed by ablation of a graphite target in a controlled atmosphere of nitrogen [3,4]. Specifically for boron–carbon–nitrogen materials, the most recent publication was from Popov et al. [25] in 1998 for the deposition of boron carbon nitride films, obtained from preheated melamine/BCl3 mixtures. Hence, variations of the laser deposition method have been developed to improve the ablation process for organic materials, examples being MAPLE (Matrix Assisted Pulsed Laser Evaporation) [26–28] and LIFT (Laser Induced

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Fig. 1. Formulas and molecular structures of the molecular carbon nitride precursors used in this work.

Forward Transfer) [29]. Here we present an improved variant of PLD that uses a femtosecond laser pulse for the ablation process and a laser-beam scanner technology, which minimizes heating and thus decomposition of the target. By using this ablation method, we demonstrate the deposition of molecular precursors of polymeric carbon nitrides, namely dicyandiamide, melamine (2,4,6triamino-s-triazine), and melem (2,5,8-triamino-tri-s-triazine). Until now, these precursors were deposited by simple sublimation methods [30–33] or direct spin-coating of the dispersed materials [34]. The crystal structures of the carbon nitride compounds consist of discrete molecules as shown in Fig. 1. These molecules are interconnected by extended hydrogen bonding to a three-dimensional network. Dicyandiamide [35] is a nitrogen-rich precursor containing two amino (NH2 ) groups as well as a nitrile (C N) feature, rendering it potentially cross-linkable. Melamine [36] and melem [37] are inert s-triazine ring systems (C3 N3 ). In melem three triazine rings are connected to a tri-s-triazine unit (C6 N7 ), and the carbon atoms at the 2, 5 and 8-position are connected to NH2 -groups. This work explores the prospect of the scanning, femtosecond-pulsed laser deposition (femto-PLD) method for the thin film formation of these precursors, a prerequisite for constructing (novel) carbon nitride networks. 2. Experimental Precursors: Dicyandiamide (C2 N4 H4 , Acros OrganicsTM , 99.5%) and melamine (C3 N3 (NH2 )3 , Carl Roth, >99%) are used without further purification. Crystalline melem (C6 N7 (NH2 )3 ) was prepared and purified from melamine following the method described by Schnick and co-workers [37]. Film preparation: Dicyandiamide, melamine and melem were deposited on polished sapphire substrates (orientation (0 0 0 1),

Table 1 Refined lattice constants of deposited films at room temperature. Dicyandiamide

Melamine

Formula Temperature Cu-K␣ ,  [Å] Space group Z

C2 N4 H4 25 ◦ C 1.54059, 1.54449 C 2/c (no. 15) 8

C3 N6 H6 25 ◦ C 1.54059, 1.54449 P 21 /a (no. 14) 4

Cell parameters [Å]

a = 15.13(1) b = 4.528(4) c = 13.18(1) ˇ = 115.59(4)◦

a = 10.60(1) b = 7.520(9) c = 7.29(1) ˇ = 112.23(6)◦

Cell volume [Å3 ] Crystal size [nm] Rwp [%]a Literature:

814.5 34 3.1

538.1 23 2.6

Cell parameters [Å]

a = 14.971(2) [35] b = 4.4918(6) c = 13.106(1) ˇ = 115.38(1)◦

a = 10.537(2) [36] b = 7.477(1) c = 7.275(1) ˇ = 112.9(1)◦

Cell volume [Å3 ]

796.3

528.0

a

Rp , Rwp and RBragg as defined in Topas Version 4.2 [40].

CrysTec GmbH) by the femto-PLD technique at substrate temperatures of −190 ◦ C or 25 ◦ C for 4–8 h. The experimental setup of the scanning femto-PLD technique has been previously described in detail [38]. The powders of all three compounds (ca. 400 mg) were ground in an agate mortar and pressed into a pellet (13 mm in diameter, 2–3 mm thick) with 10 tons. The pellet was mounted on a rotating target holder (1.5 rpm, operating distances 150 mm) inside an ultra-high vacuum (UHV) chamber and ablated by a horizontal line scan of a femto-second laser (516 nm, 442 fs) which is focused on the target surface. The laser power ranged between 30

Fig. 2. X-ray powder patterns of dicyandiamide (top), melamine (middle) and melem (bottom) deposited at room temperature onto sapphire substrates (black) compared to the calculated patterns from the crystal data (colored) (a). View of the crystal structure of dicyandiamide along the c-axis (b) and along the b-axis (c) (blue: carbon, orange: nitrogen atoms, gray lines represent the unit cell edges, COD-5900016). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 3. In situ Raman spectra of dicyandiamide deposited and measured at RT (top), and at −190 ◦ C (middle), as compared to the spectrum of the bulk measured at RT (bottom) (a). In situ Raman spectra of melamine deposited at −190 ◦ C measured directly after deposition at −190 ◦ C (middle) and after warming up to RT (top) in comparison to the bulk measured at RT (bottom) (b). In situ Raman spectra of melem deposited at −190 ◦ C (orange) and at 25 ◦ C (blue), compared to the spectrum of the bulk (black: green laser, 532 nm; gray: red laser, 632 nm). All melem spectra were measured at RT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 2 Relevant Raman and IR vibrational bands for dicyandiamide [45,46], melamine [47,48] and melem [37]. Wavenumbers (cm−1 )

Vibrational assignment

3580–3560 3380–3330 3470–3420 3185–3125 2208–2165 1660–1630 1587 1580–1555 1506 1470–1440 1254 1100–1030 1030–920 815 720 669 554 528 500

as (NH2 ) s (NH2 ) as (N–C N) ı (NH2 ) as (N–C–N) 1,3,5-s-triazine ring “quadrant stretch” as (N C–N) 1,3,5-s-Triazine ring “semicircle stretch” s (N–C N) Rock (NH2 ) s (N–C–N) 1,3,5-s-Triazine, out of plane ring bending “sextants” w (N C–N) ı (N C–N) w (NH2 )  (N–C N) r (N C–N)

and 50 mW (energy per pulse of 0.03–0.05 mJ at 1 kHz; femtoRegen IC-375, High-Q-Laser GmbH, Hohenems) which corresponds to a laser fluence of 1.2–2.0 J cm−2 . The laser beam (spot size of 0.05 mm) was scanned at a rate of 50 mm s−1 (HurryScan25, Scanlab AG, Puchheim, Samlight, Scaps GmbH, Deisenhofen). During the ablation process, the pressure in the preparation chamber was in the range of 10−6 to 10−7 mbar and the residual gas phase was

analyzed by a quadrupole mass spectrometer (QME 220, Pfeiffer Vacuum GmbH). XRD: The in situ X-ray powder patterns were recorded inside a home-made vacuum chamber on a /-diffractometer (D8Advance, Bruker AXS) with a Goebel mirror (Cu-K˛ ). The samples on sapphire substrates have a diameter of ∼10 mm and were analyzed at several locations. Details of the measurement conditions are given in a previous publication [39]. The lattice constants were refined by the Rietveld method using crystal structure data from the literature with the TOPAS software (TOPAS Vers. 4.2, Brucker AXS). Raman: In situ Raman spectra of the deposited films were recorded on a laser-microscope Raman spectrometer (iHR 550 spectrometer; BXFM microscope, manufactured by HORIBA, Bensheim) with confocal geometry. The incident laser beam (532 nm at 10 mW) passes through a window in a vacuum chamber and is focused by an objective (100×) on the samples at different positions. The Raman spectra of the powder materials were taken with a Jobin Yvon Typ V 010 Labram single grating spectrometer, equipped with a double super razor edge filter and a peltier cooled CCD camera. The resolution of the spectrometer (grating 1800 L/mm) is 1 wavenumber (cm−1 ). Spectra are taken in quasibackscattering geometry, using the linearly polarized 632.8 nm line of a He/Ne gas laser with power less than 1 mW, focused to a 10 ␮m spot through a 50× microscope objective on to the top surface of the sample. FTIR: Infrared spectra (IR) were recorded on an attenuated totalreflectance infrared spectrometer (ATR-IR, PerkinElmer, Spectrum Two) equipped with a diamond window, using a pure sapphire substrate as background.

Fig. 4. IR spectra of the dicyandiamide (a), melamine (b) and melem (c) film (colored) compared to the spectrum of the corresponding bulk material (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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SEM/AFM: Textural analyses of the thin films were carried out using a scanning electron microscope (SEM, Zeiss, Merlin) with an accelerating voltage of 1.50 kV. The film thickness and roughness were analyzed by SEM cross-section analysis. Analyses by AFM (Asylum Research: Cypher S, Oxford Instruments) were attempted, but the results were unusable due to the large height difference. XPS: X-ray photoelectron spectra were recorded by using monochromatic Al-K␣ radiation (1486.58 eV). The vacuum was kept below 1 × 10−9 mbar during the measurements. All spectra were calibrated to the C(1s) line at a binding energy of 284.6 eV. The peaks were fitted to Gaussian functions using OriginPro 8.6. The samples were transferred under ambient atmosphere into the XPS chamber and analyzed without further surface cleaning/sputtering. MALDI-TOF MS: Mass spectra were obtained in reflection mode on a Bruker Daltonics (Bremen) Reflex IV (337 nm nitrogen laser, resolution m/m = 2 × 104 ). The films were directly ablated from the sapphire substrate in positive as well as negative mode. PL: The photoluminescence (PL) spectra were measured using a Fluorolog-3 FL 3-22 (Horiba Scientific) spectrometer with a xenon lamp as light source. The emission spectra were measured between 280 and 800 nm. The sample was excited at 230 nm using a 280 nm filter to protect the detector.

3. Results and discussion Deposition of dicyandiamide, melamine and melem with femtoPLD on sapphire substrates yielded opaque films with thicknesses

in the range of 100–500 nm. In situ X-ray powder diffraction (XRD) shows that all films deposited at −190 ◦ C and at room temperature are polycrystalline. Films of all three compounds have XRD patterns largely matching those of the bulk materials (Fig. 2a). No local deviation in the homogeneity of the films was detected by XRD analyses. The lattice constants of dicyandiamide [35] and melamine [36] were refined by the Rietveld method using the single crystal data, showing modest deviation (below 1%) from the bulk data (Table 1). The obtained sizes of the crystalline domains are 34 nm for dicyandiamide and 23 nm for melamine regardless of the substrate temperature during deposition. Due to the estimated small crystallite size (ca. 4 nm) of melem [37] a reliable refinement of the lattice constants was not possible. However, the most prominent reflections of bulk melem between 25◦ and 29◦ in 2 are visible as broad reflections in the melem film. XRD patterns of dicyandiamide films present texture effects with a preferred orientation in (3 1 0) (28◦ 2, increased intensity) accompanied by an absent (0 0 2) reflection (15◦ 2). This can be explained by the crystal structure of dicyandiamide (see Fig. 2b and c) where two differently oriented layers of dicyandiamide molecules alternate along the direction of the c-axis. In these layers the molecules are arranged approximately parallel to (3 1 0) and (3 −1 0). The Raman spectra of the melamine film measured at −190 ◦ C and at room temperature directly after deposition at −190 ◦ C are in full agreement with the spectrum of the bulk material (see Fig. 3b). In case of dicyandiamide two films were measured: one deposited at −190 ◦ C and another one deposited at room temperature. The obtained Raman spectra of the films largely resemble

Fig. 5. C 1s and N 1s XPS spectra of the dicyandiamide film (bottom) compared to the corresponding powder (top).

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the spectrum of the corresponding bulk material (Fig. 3a), though in case of dicyandiamide with an increased intensity of the lattice vibration at 203 cm−1 . This feature seems to be induced by the preferred orientation of the dicyandiamide layers along (3 1 0) in the films, which raises the intensity of this lattice vibration. The same issue also influences the intensity of the X-ray reflections. Furthermore, we noticed two additional very weak, broad bands around 1500 cm−1 in the spectra of the films deposited at room temperature. It is known that in this region amorphous carbon films show similar broad Raman bands [41], which might indicate that dicyandiamide as well as melamine films start to slightly decompose during the deposition at room temperature and yield small amounts of amorphous carbon in the films. The Raman spectra of all melem films obtained (Fig. 3c) exhibit a high background. No clear Raman bands can be observed with our in situ Raman spectrometer working with a laser wavelength of 532 nm. In case of bulk melem, the Raman spectrum recorded at the same wavelength shows also a high background signal where only the most intense bands of melem (1156, 983, 548 cm−1 , compare Table 2) are observed. By using a laser wavelength of 632 nm, the background decreases for bulk melem, but not for the film samples. Thus, the Raman properties of the films vary as compared to the bulk material. The origin of the high background is given by fluorescence which is most likely caused by adventitious impurities in the films. Already a very small amount of a fluorescent material can completely cover the Raman spectra [42,43]. Furthermore, fluorescence can also be induced by partial covalent linking of melem molecules in the films, forming condensed fluorescent species [44].

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The IR spectra of all deposited samples largely coincide with the ones of the corresponding powders (see Fig. 4). The relevant vibrations for all molecular precursors are listed in Table 2. However, small deviations are observed in their peak shapes such that some weak vibrations are reduced or lost which leads to a narrower peak width (e.g., as (C N) of melamine at approximately 1400 cm−1 , the nitrile vibration at 2230 cm−1 for dicyandiamide and the (NH2 ) at 3500–2500 cm−1 in case of melem). Especially remarkable is the high intensity of the ı(NH2 ) band at 1600 cm−1 in the dicyandiamide film in comparison to the bulk material which might be related to the preferred orientation along (3 1 0) resulting in different vibrations of the terminal NH2 groups due to different hydrogen bond strengths. The carbon and nitrogen (1s) binding energies of the deposited samples and the corresponding powders were analyzed by XPS measurements (Figs. 5–7). The binding energy of C(1s) at 284.6 eV represents C–C bonds of adventitious carbon [49]. This peak is visible in all spectra, yet more prominent for the films due to the preparation method. The C(1s) binding energy around 288.0 eV is assigned to the sp2 trigonal C–N bonding (s-triazine ring) and is recorded for melamine and melem in the film as well as in the corresponding bulk materials [50]. The N(1s) XPS spectra of melamine and melem show one broad peak around 398.4–398.6 eV which represents two different binding energies, one corresponding to a two-fold sp2 C–N bond with a binding energy at 397.5–398.2 eV [50] and another one at 398.5–398.7 eV for the amine nitrogen atoms. Additionally in melem, a further peak appears around 399.9 eV (film) and 400.8 eV (powder), which can be assigned

Fig. 6. C 1s and N 1s XPS spectra of the melamine film (bottom) compared to the corresponding powder (top).

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Fig. 7. C 1s and N 1s XPS spectra of the melem film (bottom) compared to the corresponding powder (top).

to the central nitrogen of the tri-s-triazine core [50]. This more strongly shifted and broadened N(1s) signal of the film sample compared to the bulk material might be caused by condensed melem molecules in small quantities which correlate with an additional C–NH–C feature. For dicyandiamide the N(1s) binding energies of the three different nitrogen atoms are very similar. Thus, only one peak is visible in the spectra of the film and the bulk material. The C(1s) binding energies of bulk dicyandiamide show two peaks at 286.7 and 288.5 eV for the N–C N and the N–C(NH2 )2 group (besides the adventitious carbon peak). In the case of the dicyandiamide film the assignment of the individual peaks is difficult, as the peaks strongly overlap. The signal broadening can be caused by the nature of the films which do not covered completely the substrate surface with particles sizes around 100 nm.

The SEM images shown in Fig. 8 reveal the surface morphology of the obtained films of all three compounds. As shown by the top view and cross sectional micrographs, the films do not exhibit a flat surface. The thickness and roughness of the films can therefore be estimated only. The roughnesses of the films are in the same range as the film thicknesses (100–500 nm) and the particle sizes increase from 100 nm for dicyandiamide and 150 nm for melamine to 200 nm for melem, all nearly independent of the deposition temperature. The thickness of the films can be controlled by the laser power and the deposition time. At the employed power and deposition duration, we detected no significant variation on the film stoichiometry and morphology. Since the X-ray diffraction measurements of the melem films exhibit broad Bragg reflections and the Raman data show fluorescence, an additional MALDI-TOF mass spectrum of the

Fig. 8. SEM images of the surface of dicyandiamide (a), melamine (b) and melem (c) films on sapphire substrates.

D. Fischer et al. / Applied Surface Science 349 (2015) 353–360

Fig. 9. MALDI-TOF mass spectra of the melem film (cyan) and the corresponding powder (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

melem sample was recorded and compared to the bulk material (Fig. 9). The MALDI-TOF results demonstrate that the melem film consists of single units of melem and partly “dimelem” (di-tris-s-triazinylamine), whereas the bulk material contains almost entirely melem. Since no obvious C–NH–C vibrations are visible in the IR fingerprint region of the melem film, dimelem or further melem-oligomers can only have been formed with rather small yields. Furthermore, PL measurements of the thin films and the corresponding bulk material demonstrate that all three carbon nitride precursors show photoluminescence in the UV region around 390 nm (Fig. 10). Whereas the photoluminescence spectra of the melamine and melem films are the same as their bulk counterparts, the dicyandiamide film shows an additional shoulder at slightly higher wavelengths. This shoulder might indicate partial condensation of the dicyandiamide molecules to larger oligomers. However, it seems that the peak emission wavelengths are largely the same for the bulk as compared to the film samples. This observation supports the conclusion that the films match the bulk material and that no significant chemical changes are present. The rather weak intermolecular interactions (hydrogen bonds) of the molecules cause the deposition to be independent of temperature. Thus, deposition of the molecules was achieved at low temperature (−190 ◦ C) as well as at room temperature with equally crystalline films. The weak intermolecular interaction of the molecules leads to an extensive vapor-zone during the ablation process and a broad distribution of the ablated species inside the vacuum chamber which would benefit the inverse PLD technique, where the backside motion of the ablated species is utilized for film growth [51]. With the inverse PLD geometry, placing the substrate in the target plane, the growth of carbon nitride films has been investigated under different experimental conditions. Despite the

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broad distribution of the molecules, only a thin layer of the materials is deposited on the substrate which indicates a low sticking coefficient of the particles on the sapphire, or follow-up hits of the molecules on the deposits, especially at higher temperatures. These follow-up hits seem to slightly damage the deposits at room temperature which can be monitored in XPS and Raman measurements by the increased adventitious/amorphous carbon peak of the films as compared to spectra of the bulk material. Furthermore, we noticed that the ablation of larger and heavier molecules such as melem is more challenging compared to smaller ones like dicyandiamide, which becomes apparent in smaller crystallite domain sizes for melem compared to dicyandiamide and further linking of the melem molecules in small yields. In contrast, the small molecular size of dicyandiamide and its layered structure cause a high mobility of the molecules on the sapphire substrate, such that texture effects are visible in the X-ray powder patterns. 4. Summary and conclusions We presented the femto-second pulsed laser deposition as a viable technique for depositing polycrystalline thin films of molecular precursors for carbon nitride, namely dicyandiamide, melamine, and melem, on sapphire substrates. The key parameters for the deposition, such as substrate temperature and deposition duration have been investigated. Characterizations of the films show that in all aspects the films largely resemble the bulk material, demonstrating that highly heat-sensitive molecules have been deposited by femto-PLD. These results open the possibility to form thin films of temperature-sensitive or reactive materials that are difficult to deposit in general, in particular compounds based on carbon nitride networks. The successful ablation of these heat-sensitive molecules bodes well for the growth of thin films of molecular compounds on arbitrary substrates, which will be relevant e.g. for applications in optoelectronics such as thin film solar cells or LED devices composed of organic compounds. Acknowledgments We thank Dr. M. Konuma (Interface Analysis, MPI for Solid State Research, Stuttgart) for the XPS analysis, A. Schulz (Keimer Department, MPI for Solid State Research, Stuttgart) for the Raman measurements of the powder materials, and V. Duppel (Nanochemistry Group, MPI for Solid State Research, Stuttgart) for the SEM images. K. Schwinghammer and B. V. Lotsch thank for the financial support by the Deutsche Forschungsgemeinschaft (projects LO1801/1-1), the cluster of excellence “Nanosystems Initiative Munich” (NIM), and the Center for NanoScience (CeNS).

Fig. 10. PL spectra of the dicyandiamide (a), melamine (b), and melem (c) films (colored) compared to the corresponding bulk material (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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