Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride

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C: Physical Processes in Nanomaterials and Nanostructures

Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride Nanosheets Probed by Positron Annihilation and Photoluminescence Spectroscopy Biswajit Choudhury, Kamal Kumar Paul, Dirtha Sanyal, Anil Hazarika, and Pravat Kumar Giri J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01388 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 8, 2018

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The Journal of Physical Chemistry

Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride Nanosheets Probed by Positron Annihilation and Photoluminescence Spectroscopy Biswajit Choudhury1*, Kamal Kumar Paul2, Dirtha Sanyal3,4, Anil Hazarika5, P. K. Giri2 1

Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Guwahati 781035, India 2

Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India 3 4

Variable Energy Cyclotron Center, 1/AF Bidhanngar, Kolkata 700064, India

Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India

5

Department of Electronics and Communication Engineering, Tezpur University, Tezpur 784028, India

*Corresponding Author: [email protected] , [email protected] , Phone: +918486912412

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ACS Paragon Plus Environment

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ABSTRACT: Defects play a pivotal role in the device performance of a photocatalytic, light emitting or photovoltaic system. Herein, graphitic carbon nitride (g-C3N4) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect bound excitonic emissions peaked at 2.78 eV, 2.58 eV and 2.38 eV are dominant with above bandgap excitation. Time-resolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect bound exciton. We provide a direct correlation between the defects observed by PAS with that of the excitonic lifetime found from the PL studies. Below bandgap excitation activates defect emission and it is characterized by a short carrier lifetime (~0.14 ns). Excitation power-dependent PL study with 405 nm laser shows progressive red-shift and narrowing of the emission line. We have interpreted the different PL features with defect band-filling of exciton, interplanar, intraplanar, interchain exciton migration, etc. These results are significant for tuning the optoelectronic properties of g-C3N4 nanosheets and exploiting its applications in various emerging areas.

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1. INTRODUCTION 1

Carbon nitride with graphene-like layered structures has drawn full attention for water splitting

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reaction and in the decontamination of aqueous organic pollutants under visible light.1-4 The

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system has a bandgap of 2.7 eV and builds up of s-heptazine units with tertiary nitrogen bridging

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the nearest heptazine structures.1,3 Graphitic carbon nitride (g-C3N4) can be synthesized by

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simple thermal condensation method from low-cost precursors, viz., thiourea,5 melamine,6

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cyanamide,7 urea,3,8, etc. Temperature has a direct influence on the degree of condensation and

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packing between layers, which otherwise influences the π conjugation in the sp2 bonded

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framework.9 The high calcination temperature is known to reduce the interlayer distance in C3N4.

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Compactness of the layers has an impact on the spectral red-shift in absorption and

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photoluminescence. Wu et al. with density functional calculation studied the effect of interlayer

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electron coupling on the optical properties of C3N4.10 In another study, Chen et al. reported that

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the steric interactions of the lone pair electrons on adjacent nitrogen atom could tune the visible

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light absorption in C3N4.11 C3N4 emits strong blue light under UV excitation.12 Its strong

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excitonic emission in the visible region, thermal resistance, and tunable bandgap makes C3N4 a

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popular choice for light-emitting diodes, luminescence-based bio, and chemosensor.13-16 Excitons

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generated in the heptazine unit of C3N4 are reported to undergo facile migration along π stacking

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direction in C3N4.17 Long distance exciton transport is necessary for photovoltaic and light

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emitting diode applications.18-19 C3N4 quantum dots incorporated into polymer cells show

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superior solar cell performance wherein the π conjugated system of C3N4 acts as charge transport

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medium in the hybrid solar cell.19 Cui et al. observed sufficiently long excitonic lifetime in

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phenyl modified graphitic carbon nitride and its possible requirements for advanced optical

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super-resolution techniques.20 3



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Defects are inevitable in C3N4 as its structure evolves through the formation of several

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intermediates, such as cyanuric acid, melon, etc.21 The various uncondensed defects in C3N4 are

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–NH2/NH, cyano/cyanide (C≡N, -NCN-) and oxygenated functional groups (-OH, -COOH).21,22

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Besides, intrinsic defect such as N and carbon vacancies are also generated in C3N4 in the

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thermal condensation process.23-25 Dong et al. observed efficient N2 fixation ability of N

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deficient carbon nitride.23 Similarly, Tay suggested positive attributes of a defect in enhancing

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photocatalytic H2 production in C3N4.24 With surface photovoltage spectroscopy (SPV), Wu and

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his team measured the presence of NH/-NH2 related shallow and deep trap states in the bandgap

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of C3N4.26 It was speculated that these defects are detrimental to the efficient production of H2.

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In the present study, we aim to understand the specific nature and evolution of defects in

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C3N4 as a function of its calcination temperature by using positron annihilation spectroscopy

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(PAS) and photoluminescence (PL) spectroscopy techniques. We investigate the influence of

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structural defects on the excitonic emission in C3N4. A detailed understanding of the nature of

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defects in C3N4 and its impact on the excitonic luminescence and decay processes is necessary

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before the implementation of this material in optoelectronics, photovoltaics, and solar cell

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applications. Here, we employ steady-state and time-resolved spectroscopy techniques to identify

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the free excitonic, bound excitonic and defect-related emissions and their decay dynamics. C3N4

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are grown at different calcination temperatures. To the best of our knowledge and understanding,

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for the first time, we have employed PAS to identify the nature of defects in C3N4 nanosheets

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and provided a direct correlation between results of PAS and PL. PAS is a unique nuclear solid-

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state technique to study the electron density and electron momentum distribution and in

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particular, vacancy-type defects in the material.27-28 Free and defect bound excitonic emission

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and trap state related luminescence and the associated carrier lifetime in C3N4 are studied with 4


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photoexcitation energies above and below the optical bandgap of the samples. Our results show

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that defect bound excitons have sufficiently longer lifetime than that of the free excitons.

48 49

2. EXPERIMENTAL

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2.1.

Preparation of graphitic carbon nitride (g-C3N4) at various temperatures

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Graphitic carbon nitride was synthesized by thermal condensation method. Typically, urea was

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taken in a beaker and heated in a reaction hood till it was scorched. The dried powder was

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crushed in a mortar to get fine white powder. The powder was taken in a crucible covered with a

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lid and heated in a tube furnace at 420 oC for 2 h. The final product was whitish yellow which

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was again finely ground and kept in a sample vial. Similarly, calcination of dried urea was

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performed at 470 oC, 520 oC, 570 oC, respectively.

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2.2.

Materials Characterisation

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X-ray diffraction (XRD) was performed in a Bruker D8 Advance X-ray diffractometer (0.154 nm

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Cu Kα radiation, 40 kV, 40 mA) for the diffraction range 2θ~10-35o. Low and high-resolution

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transmission electron microscope (TEM) images were collected in FEI Technai G2 20 S-Twin.

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The operating voltage was 200 kV, and the spectral resolution was 2.4 Å. Fourier transform

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infrared (FTIR) spectroscopy data were recorded in a Perkin Elmer, Spectrum BX

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spectrophotometer. TGA was performed in a Perkin Elmer TGA 4000 series. The measurement

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was performed under N2 flow at a heating rate of 10 oC/min. Diffuse reflectance spectra (DRS)

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of the samples were collected in a Shimadzu UV–visible spectrometer (Model 2450) with BaSO4

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as the standard reference. Steady-state photoluminescence spectra of the samples were monitored

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in Fluoromax-4, Horiba Scientific spectrometer with an excitation and emission slit width of 2

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nm. Laser power dependent PL studies were performed using a 405 nm (3.06 eV) diode laser 5



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(Cube, coherent) equipped with cooled charge-coupled (CCD) detector (Princeton instruments,

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PIXIS 100B). Time-resolved photoluminescence (TRPL) spectra were recorded using a

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picosecond time-resolved luminescence spectrometer (Edinburgh Instruments, model: FSP920).

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The chemical compositions of the samples were evaluated with X-ray photoelectron

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spectroscopy (XPS) PHI X-tool automated photoelectron spectrometer (ULVAC-PHI, Japan).

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The source was Al Kα X-ray beam source (1486.6 eV), and the recorded beam current was 20

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mA. In XPS measurement, adventitious carbon (AdC) C 1s signal at 284.8 eV is considered as

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binding energy (B.E.) reference.29,30 Au 4f signal might be considered as a B.E. reference peak

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for carbon materials to reduce the error (0.1±0.2) while considering C 1s as reference.29,30

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However, correct referencing of Au 4f signal for B.E. correction is not properly resolved and

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thus, C 1s referencing is considered as widely adopted method for B.E. correction.29 Therefore,

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in consensus with the report,30 we have considered C 1s at 284.8 eV for binding energy (B.E.)

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corrections of different elements in C3N4. Defects were identified with positron annihilation

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spectroscopy (PAS). In the present study, coincidence Doppler broadening (CDB) experiments

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with two identical HPGe detectors of 12 % efficiency (Type, PGC 1216sp of DSG, Germany)

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having energy resolution of 1.15 at 514 keV of 85Sr was used in coincidence. A 10 µCi22 NaCl

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source encapsulated in a 1.5 micron-thin nickel foil was used as a positron source. The details of

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the experimental set-up were mentioned elsewhere.31 The total counts under the CDB 511 keV

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spectra of about 107 had been recorded in a dual ADC-based multiparameter data acquisition

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system (MPA-3 of FAST ComTec., Germany). The CDB spectra had been analyzed by

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constructing the area normalized ratio-curve method.

90 91 6


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3. Results and discussion

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Figure 1 shows the XRD pattern of g-C3N4 processed at different temperatures. For all the

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samples we have observed two diffraction peaks corresponding to (002) (2θ = 27o) and (100)

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crystallographic planes (2θ = 12.6o).3 The shifting of (002) peak to higher diffraction angle with

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calcination temperature is shown in the inset of Figure 1. Calculated d-spacing results for the

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(002) and (100) planes at various calcination temperatures are inserted in Table S1. A reduction

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in the d-spacing with calcination temperature for (002) plane indicates tight packing of layers

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along the π-stacking direction of aromatic g-CN units. A reduced stacking distance will increase

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the repulsion of the lone pair on aromatic as well as on bridging N atoms, favouring buckling of

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nanosheets at higher calcination temperatures.32,33 As for the (100) peak, the d-spacing slightly

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increases with calcination temperature. An increase in d-spacing indicates a larger intraplanar

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distance of s-heptazine unit resulting from steric repulsion of lone pair electrons present on

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neighbouring nitrogen atoms in s-heptazine unit.32,34

105 106 107

Figure 1 XRD pattern of C3N4 synthesized at different calcination temperatures. Inset shows the expanded view of (002) peak revealing clear peak shift. 7



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Figure 2 displays TEM images of C3N4 grown at the different calcination temperature. As seen in

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the low-resolution TEM images of CN420, CN470, CN520, and CN570 in Figure 2a, 2c, 2e and

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2g, the as-grown C3N4 nanosheets at different temperatures acquire corrugated morphology with

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porous structures. The corrugation in the nanosheets is believed to result from the repulsion of

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lone pair electrons on bridging nitrogen in the s-heptazine unit of the adjacent layers.35 The pore-

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like features can be well visualized in the high-resolution images of the samples (Figure 2b, 2d,

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2f, and 2h). High-resolution images show that the nanosheets contain tightly packed layers with

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approximately ~10-18 layers in the condensed thick structures of the nanosheets. FTIR spectra

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(see in Figure S1, supporting information) of the samples reveal different types of C-N bonding

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or presence of uncondensed CN, -CNH in the system. The broad absorption band between 3700-

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3000 cm-1 comes from –NH vibration, -OH vibration, etc.36,37 The fingerprint region (1705-800

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cm-1) of g-C3N4 contains several absorption bands. These bands are associated with symmetric,

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asymmetric, bending vibration of aromatic and bridging C-N (or C=N) bonds in the s-heptazine

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units of C3N4.38,39 Thermogravimetric analysis (TGA) of C3N4 prepared at different calcination

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temperatures are shown in Figure S2 (supporting information). For CN420, there is 7 % weight

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loss till temperature reaches 200 oC, and from 200-450 oC the weight loss is 2 %. In case of

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CN470, CN520 and CN570 the total weight loss is 6 %, 3.5 %, and 0.5 %, respectively, till

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temperature reaches 470 oC. The initial weight loss up to 200 oC is due to the removal of

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adsorbed water and the final weight loss is due to the decomposition of intermediates, such as

127

cyanuric acid, ammeline, and ammelide, formed during the condensation of C3N4.40 The total

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weight loss for the samples is very low, which indicates that C3N4 is the stable species with

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fewer impurities. The rapid weight loss above 470 oC is due to the removal of the nitrogenated

8


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impurities and gases.41 Above 650 oC there is more than 95 % weight loss which is due to the

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decomposition of C3N4.40

132 133 134

Figure 2 Low- and high- resolution TEM images of: (a-b) CN420, (c-d) CN470, (e-f) CN520 and (g-h) CN570.

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The chemical composition of as-prepared C3N4 nanosheets is studied with X-ray

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photoelectron spectroscopy (XPS). The C 1s XPS spectrum for CN520 is shown in Figure 3a,

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and those of other samples are shown in Figure S3. In CN520, C 1s peak at 288.1 eV can be

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assigned to sp2 hybridized N-C=N in aromatic s-heptazine ring, and the shoulder peak at 284.7

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eV is arising from the adventitious carbon.5,42,43 The weak C 1s peak at 286.2 eV is probably

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occurring due to the presence of nitrogen-related defects in the graphitic carbon nitride system

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and attributed to N-CH=N bonding structure in the system.43 The high-resolution N 1s spectrum

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for CN520 is deconvoluted into three components with peaks centered at 398.4 eV, 400.1 eV,

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and 404.1 eV, respectively (Figure 3b). N 1s spectra for rest of the samples are included in

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Figure S3. The dominant peak at 398.4 eV can be ascribed to two coordinated N atom (N2C in C-

145

N=C in aromatic ring), and the shoulder peak at 400.1 eV is attributed to three coordinated N 9



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atom (N3C, tertiary nitrogen N-(C3) groups).5,23 The additional minor peak at 404.1 eV can be

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assigned to cyano group or π-excitation.37 We measured the peak-area ratio of N2C to N3C for

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C3N4 calcined at different temperatures. It is seen that N2C/N3C ratio changes as 2.14 (CN420),

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2.36 (CN470), 0.83 (CN520) and 0.92 (CN570). A lower value of N2C/N3C indicates the presence

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of nitrogen-vacancy at the N2C site, and from the measured values it is evident that CN520

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experiences higher nitrogen loses at the two coordinated sites (N2C).25, 42,43 This implies a higher

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density of nitrogen-vacancy defects in CN520.

153

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Figure 3 Core level (a) C 1s and (b) N 1s peak XPS spectrum of CN520

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To support our prediction from XPS that CN520 contains higher nitrogen-related vacancy

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centers, we have employed positron annihilation spectroscopy (PAS) technique. PAS is a

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powerful tool to identify vacancy-type defects in a material.

10


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Figure 4 (a) Ratio of area normalized CDB spectra of calcined C3N4 with respect to area normalized CDB spectra of 420 oC calcined C3N4. (b) Doppler broadening shape parameter, Sparameter, for differently calcined C3N4 samples.

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We have considered coincidence Doppler broadening (CDB) method to determine the

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nature of defects in C3N4.44 Figure 4a represents the ratio of area normalized CDB spectra of

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calcined (470, 520 and 570 oC) C3N4 samples concerning the area normalized CDB spectra of

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C3N4 calcined at 420 oC. It is clear from the Figure 4a that due to calcination at a higher

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temperature, i.e., 520 oC and 570 oC, the ratio curve shows a broad dip in the momentum value

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p L ~ 10×10-3 m 0c . The dip around p L = 10×10-3 m 0c represents more open volume defects in the

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sample since the core electron wave functions are localized and do not span inside the available

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volumes.31 One can identify the core electrons with which positrons are less annihilating by

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estimating the kinetic energy of the electrons corresponding to the p L ~ 10×10-3 m 0c . Positrons

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are almost thermalized before annihilation and using Virial theorem approximation (in the atom

172

the expectation value of the kinetic energy of an electron, is equal to the binding energy of the

173

electron), one can calculate the E kin using p L = (2m 0 E kin ) 2 .45 The calculated kinetic energy

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corresponds to p L ~ 10×10-3 m 0c ~ 26 eV. In C3N4 nanosheets the core electron near to this value

175

is the only 2s electron of the nitrogen atom (~37 eV). This shows that due to calcination at 520

1

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0

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nitrogen-vacancy is less in case of 420 0C and 470 0C calcined samples.

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The Doppler broadening shape parameter, S-parameter has been calculated for all the calcined

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C3N4 samples. The S-parameter of the Doppler broadening of the positron annihilation gamma-

180

ray line profile N(E) vs. E is defined as the ratio of the counts in the central area

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(511 keV − E ≤ 0.85 keV)

182

parameter represents the fraction of positrons annihilating with the “low” momentum electrons to

183

the “higher” momentum electrons. An increase of S-parameter indicates the increase of defects

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in the system.31,45 Figure 4b depicts an enlargement of S-parameter due to calcination at 520 0C

185

and 570 0C sample. It is also clear from Figure 4b that the maximum defects can be generated in

186

the sample calcined at 520 0C, which is in agreement with the ratio curve analysis.

C, a large density of nitrogen vacancies has been created in the C3N4 sample, but the density of

(

and the counts in the total area 511 keV − E ≤ 4.25 keV ) . Thus S-

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Figure 5a shows the Kubelka-Munk absorption plots of g-C3N4 prepared at different

188

calcination temperatures. The corresponding reflectance plot of the samples is shown in Figure

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S4a. The samples show two absorption bands centered at 277 nm and 378 nm. These bands are

190

attributable to π-π* transition of the aromatic sp2 C=N unit and n-π* transition involving

191

nonbonding electrons on N atom in aromatic sp2 (C=N) and bridging sp3 (C-N).46,47 The bandgap

192

of the samples is determined from the plot of [F(R)hv]1/2 vs. hν (Figure 5b,Table S1). The

193

bandgap in C3N4 is a measure of indirect optical transition from Г to M in the Brillouin zone.48 It

194

is seen from table S1 that the bandgap of the samples decreases with an increase in calcination

195

temperature up to 520 °C. Up to this temperature the bandgap variation follows the trend 2.83 eV

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(CN420), 2.75 eV (CN470) and 2.73 eV (CN520). The bandgap again increases to 2.85 eV in

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CN570. The decrease in the bandgap can be attributed to the presence of intermediate defect 12


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states, such as uncondensed NH/NH2, or structural defects such as nitrogen vacancies.24,26 The

199

enlargement in the bandgap in completely grown CN570 is attributable to quantum confinement

200

effect. The intermediate defect states constitute the Urbach energy (Eu) tail below the optical

201

bandgap of the samples.

202 203 204

Figure 5 (a) Kubelka-Munk absorption plots, and (b) Tauc plots for the band gap calculation for C3N4 samples calcined at different temperatures.

205

We have determined Urbach energy for each sample from the slope of the linear fitting

206

between lnF(R) vs. hν.49 The measured values of Eu for different samples are included in Table

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S1. There is an inverse relationship between Eg and Eu (Figure S4b). The samples with higher

208

bandgap have lower Eu and vice-versa. A higher Urbach energy value indicates the presence of

209

intermediate defect states in the samples. These defect states are mostly nitrogen-related as

210

manifested by the results of PAS. The overall trend of variation of S-parameter and the variation

211

in the bandgap with calcination temperature is more or less correlated, although not quite

212

matched. This is expected since positron probes only defects. However, for determining the

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bandgap, defect is one of the many parameters. There are other parameters which might

13



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influence the bandgap modification in C3N4. These include interlayer electron-electron coupling,

215

conjugation of π electron system, interlayer periodicity, etc.10,50, 51

216 217

Figure 6a shows the steady-state PL spectra of as-grown C3N4 nanosheets under an optical excitation energy 3.35 eV, well above the samples bandgap.

218 219 220 221

Figure 6 (a) PL spectra of C3N4 nanosheets grown at different temperatures under an excitation energy of 3.35 eV. The inset shows a magnified view of the PL spectrum of CN420. (b) PL spectra of CN520 obtained under different excitation energies.

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The spectrum has a maximum emission in the blue region and a low energy tail extending down

223

to 1.8 eV. The blue emission is due to exciton generated in the s-heptazine ring.52 The excitonic

224

emission is red-shifted by ~0.08 eV as the calcination temperature increases from 420-470 °C

225

(Figure S5, supporting information). No change in the position of emission energy is monitored

226

at 520 °C and 570 °C. The observed spectral red-shift might be corroborated by the structural

227

changes in the as-grown C3N4. The changes include degree of condensation of the layers during

228

growth of C3N4, extrinsic and intrinsic defect formation, such as –NH/-NH2 surface

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decomposition, an aromatic ring and bridging N defect formation, etc.43,46,53 Growth temperature

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also affects the line-width of an excitonic emission. There is an increment in full-width at half 14


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maximum (FWHM) by ~0.1 eV on increasing the calcination temperature from 420-470 0C, as

232

seen in Figure S5. Above 470 0C and up to 570 0C, there is a marginal increase in the PL line-

233

width. The structural changes might also be responsible for the broader distribution of emission

234

centers, resulting in the broadening of emission peak.14,54 DFT based theoretical study has also

235

reported that PL properties of C3N4 exhibit extensive modifications in the presence of defects.

236

This has resulted in a synergetic effect of interlayer periodicity and formation of structural

237

defects.50 Since interlayer distance diminishes with increasing calcination temperature (as

238

revealed from the XRD analysis), possible exciton-exciton coupling and scattering might also

239

contribute to the broadening of the PL peak.55 We have also measured the impact of excitation

240

energy variation on the excitonic emission of C3N4. Figure 6b shows the PL spectra of CN520

241

under excitation energies of 3.35 eV, 3.54 eV, 3.75 eV, 4.00 eV and 4.27 eV. There is a

242

monotonic decrease in PL intensity with increasing excitation energy, which may be due to a

243

decrease in intensity of excitation light (a Xenon lamp) at higher energy (from near visible to

244

UV).56 Note that the position of the blue PL band is almost unchanged irrespective of the

245

incident excitation energy. Non-dependence of emission energy on excitation energy suggests

246

that excitonic emission occurs from the same excited π* state irrespective of probing energy.57

247

The emission spectrum of C3N4 (Figure 6) has a low energy tail emission

248

alongside the strong excitonic emission. To extract different emissive components from the

249

broad PL spectrum, we have deconvoluted the PL spectrum of CN520 with three Gaussian

250

peaks. The peaks are centered at 2.78 eV, 2.58 eV, and 2.38 eV, respectively (Figure 7a). In

251

conformity with the bandgap values determined for the absorption spectra and in agreement with

252

literature report, we infer that the intense emission peak at 2.78 eV is the excitonic peak.52 Other

253

emission peaks at 2.58 eV and 2.38 eV are lower than the measured bandgap of the samples. 15



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Therefore, these emissions cannot be related to the free excitonic emission. We speculate that

255

these excitonic emissions are associated with structural defects in C3N4 nanosheets.

256 257 258 259 260 261

Figure 7 (a) Decovoluted (Gaussian) PL spectrum of CN520 under excitation of 3.35 eV. (b) TRPL (logarithmic plot) spectra of CN520 excited at 3.30 eV and monitored at 2.78 eV, 2.58 eV and 2.38 eV emission energies. (c) Plot showing a relation of S-parameter (left Y-axis) and average bound excitonic lifetime (τav at 2.38 eV) (right Y-axis) as a function of calcination temperature, which demonstrates a direct correlation between the S-parameter and τav.

262

263

For a detailed understanding of the associated PL decay, we have recorded the TRPL spectra for

264

different samples at the deconvoluted emission energies. TRPL spectra for CN520 at emission

265

energies 2.78 eV, 2.58 eV and 2.38 eV are obtained under excitation energy of 3.3 eV, and the

266

results are shown in Figure 7b. The decay profile is recorded for 0-200 ns time scale and the

267

amplitudes and lifetime components are listed in Table 1. It is found that the excitonic emission

268

at 2.78 eV has an average lifetime of 17.4 ns, whereas for the 2.58 eV and 2.38 eV emission

269

peaks the lifetimes are extended to 25.2 ns and 27.4 ns, respectively. Therefore, this extended 16


Page 17 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


270

carrier lifetime may be due to the involvement of bound excitons in the transition. The TRPL

271

spectra for the other samples are shown in Figure S6. From the PAS analysis, we have found that

272

S-parameter measures the nitrogen-vacancy defects in the system. Higher the S-parameter,

273

higher is the concentration of defects in the system. Figure 7c shows a comparison of the average

274

lifetime (τav) obtained for different samples for the bound excitonic emission. It is seen that

275

CN520 which has the highest defect content (the highest S-parameter) also possesses the longest

276

average PL lifetime for bound excitonic emission. Thus, a direct correlation between the vacancy

277

type defects and the excitonic lifetime established through this data allows us to unambiguously

278

identify the corresponding emissions as bound excitonic emissions in C3N4.

279 280

Table 1. TRPL Data of CN520 Excited at 3.3 eV and Monitored at 2.78 eV, 2.58 eV and 2.38 eV

Emission Energy. Sample

Emission (eV)

Lifetime (ns)

τ 1 (a1 )

CN520

τ 2 (a 2 )

τ av (ns) τ 3 (a 3 )

2.78 eV

1.1 (0.22)

5.0 (0.53)

23.6 (0.25)

17.4 ns

2.58 eV

1.9 (0.29)

7.0 (0.46)

33.6 (0.25)

25.2 ns

2.38 eV

1.8 (0.25)

6.9 (0.48)

36.0 (0.22)

27.4 ns

281

282

From the three components ( τ1 , τ 2 , τ3 ) of the excitonic lifetime in Table 1 for CN520, it

283

is clear that τ1 component does not change much for the three emission peaks. It is τ 2 and mostly

284

the τ3 component, which shows a large difference from that of free excitonic emission. It is seen

285

that the excitonic emission has an average lifetime of 17.4 ns, whereas for the 2.58 eV and 2.38

286

eV emission peaks the lifetimes are extended to 25.2 ns and 27.4 ns, respectively.

287

Now we elaborate on the origin of different lifetime components ( τ1 , τ 2 , τ3 ) for free 17



Page 18 of 44

288

excitonic emission at 2.78 eV. Figure 8 gives a schematic view of different pathways of exciton

289

migration.

290 291 292 293 294

Figure 8 Schematic shows exciton migration in (a) π-stacking direction (interplanar), and (b) intraplanar migration (1) and intrachain (2) migration. Free excitons (F-exciton), when attached to nitrogen related defects (N-defects) create bound excitons (DB-exciton). Exciton migration is prohibited if C-H or N-H moiety terminates the heptazine link.

295

After photoexcitation, excitons that are confined to the aromatic ring undergo fast recombination

296

and gives rise to the fast decay component ( τ1 ). Excitons generated in the s-heptazine might also

297

migrate in the extended s-heptazine rings of C3N4 following different pathways. Excitons might

298

undergo facile interplanar migration along a π-stacking direction (Figure 8a) and transport from

299

one layer to another giving rise to the long exciton lifetimes ( τ 2 and τ 3 ).17 The other possible 18


Page 19 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

pathways of exciton migration that could result in the long excitonic lifetime could be as

301

depicted in Figure 8b. In interplanar migration, exciton moves between two connected s-

302

heptazine units with different symmetry planes. In intrachain migration exciton moves along the

303

s-heptazine chains through bridging N atom giving long excitonic lifetime, however, considering

304

its migration is not interrupted by the presence of N-H bonds or C-H bonds. Thus, we have

305

considered interplanar, intraplanar and intrachain migration as the possible pathways for

306

migration that could result in the long components of the free excitonic lifetime ( τ 2 and τ 3 ).

307

However, reports show that exciton migration follows an exponential decrease with stacking

308

distance.52 Table S1 shows that interplanar (or pi-stacking) distance of adjacent layers in C3N4

309

are separated by ~3.2 A . However, for intraplanar migration or migration along the heptazine

310

chains the exciton sitting on the nearest s-heptazine are separated by ~6.9 A (Table S1).

311

Therefore, exciton coupling and migration are supposed to be weak in this direction. Therefore,

312

we can predict that intraplanar and intrachain exciton migration can be responsible for the

313

τ 2 component. However, it is the interplanar or π-stacking direction which is sought to be the

314

most suitable pathway for long transport of exciton in C3N4 giving the longest τ3 component. If

315

we compare the different components ( τ1 , τ 2 , τ3 ) of lifetime for free excitonic (2.78 eV) and

316

bound excitonic emission (2.58 eV and 2.38 eV), we can easily interpret that it is the

317

τ3 component which is quite long in case of bound exciton emission compared to free emission.

318

Excitons bound to defect are relaxed for a longer period in the defect states before it finds a

319

recombination center. In this way, the defect bound excitons show high fluorescence stability

320

over the longer timescale.

0

0

19



Page 20 of 44

321

To understand the response of the defect bound excitons to laser power excitation, we

322

have taken PL spectra of CN520 under diode-pumped solid-state laser. Figure 9a shows the

323

normalized PL spectra of CN520 under excitation of 3.06 eV (405 nm).

324 325 326 327 328

Figure 9 Normalized PL spectra of CN520 for different excitation power (intensity). The samples are excited with a solid-state laser of energy 3.06 eV. (b) Changes in emission energy (left Y-axis), full width at half maximum (FWHM) (right Y-axis) with laser power. The inset shows the PL emission intensity as a function of excitation laser power.

329

The emission spectra are recorded under laser power of 5 mW, 10 mW, 15 mW, 20 mW

330

and 25 mW. The un-normalized PL spectrum is shown in Figure S7. The emission intensity,

331

position and line-width display clear dependence on incident laser power, as shown in Figure 9b.

332

First, we will emphasize the changes in emission intensity with probing laser power. As shown

333

in the inset of Figure 9b, the emission intensity increases slightly on increasing the laser power

334

from 5 mW to 10 mW. There is a significant enhancement in PL intensity at a laser power of 15

335

mW. However, saturation of PL intensity occurs at higher excitation power (>15 mW). From the

336

previous study, it is understood that the emission intensity (IPL) of a sample exhibits a linear

337

variation with incident laser power following the relation, I PL ∞Lα , where L is excitation power

338

and α is power factor.58 In case of free excitonic emission, the emission intensity scales linearly

339

with laser power, whereas for bound excitonic emission the intensity varies linearly at low 20


Page 21 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


340

excitation power and deviates from linearity and saturates at high laser power.59 Therefore, we

341

might speculate that the non-linear intensity variation and saturation is resulted by these defect

342

bound excitons of C3N4. The position of emission energy is unchanged for the laser power of 5

343

mW and 10 mW (Figure 9b). There is a red-shift by ~0.5 eV when the laser power is set at 15

344

mW. On increasing the laser power from 15 mW to 20 mW, there is a shift of ~0.1 eV, and in

345

between 15 mW to 25 mW the shifting is ~0.2 eV. A similar trend is observed for FWHM

346

variation with laser power. Several factors have been associated with the red-shift of PL band

347

including laser heating effect, defect level population with photoexcited carriers, bandgap

348

renormalization, energy transfer, etc.58,60 Laser heating effect is prominent in nanoparticles

349

where heat conduction among nanoparticles is poor, and the heating effect is localized on a

350

single nanoparticle.61 However, in C3N4, having a conjugated 2D network, the heat energy might

351

go easy transfer throughout the system and therefore, localized heating effect as such may not be

352

prominent. Moreover, laser-induced local heating is known to cause spectral red-shift and

353

quenching in emission intensity by enhancing electron-phonon interaction and increasing non-

354

radiative recombination centers.62,63 No such spectral features have been observed in the PL

355

spectra. Therefore, laser heating is not the principal reason for the changes observed in emission

356

profile. The observed changes in PL feature can be associated with defect level population with

357

carriers.64 Since the system is excited with 3.06 eV laser, which is above the bandgap of the

358

samples, bound excitons are generated in the system. The number density of such excitons shows

359

dependence on incident laser power. At incident laser power of 5 mW and 10 mW, less number

360

of excitons could occupy the defect states near the conduction band edge. As laser power is

361

increased to 15 mW, large numbers of excitons could fill up the defect spaces distributed near

362

the band edge. Exciton distribution is expected to be non-uniform, and the number of radiative 21



Page 22 of 44

363

recombination centers participating in luminescence appeared to be very high giving the spectral

364

broadness or increase in FWHM (Figure 9b). However, when the laser power is increased to 15

365

mW and 20 mW, the upper-level population saturates, and the excitons start to fill-up underlying

366

defect levels. Many of the excitons undergo exciton-exciton scattering and decay non-

367

radiatively. Only those excitons which can occupy the defect levels contribute to luminescence.

368

In the lower lying defect levels the exciton distribution appears to be uniform, and since the

369

number of radiative emission centers in these states seems to be less, the emission spectra attain

370

a narrow and symmetric feature.

371

C3N4 displays defect related sub-bandgap emission when the samples are photoexcited

372

with energies below the optical bandgap. In C3N4 the types of defects are nitrogen related, and

373

these could be cyano (C≡N), NH2, nitrogen vacancy, etc. PAS has characterized presence of

374

these nitrogen related defects. Figure 10a shows the normalized PL spectra of the samples

375

excited at 2.69 eV, 2.61 eV, 2.53 eV, 2.45 eV and 2.38 eV. Unlike above bandgap (BG)

376

photoexcitation where there is a band-to-band optical transition, photoexcitation below BG

377

triggers band to defect (D) transition, viz. π→D or LP→D. A broad asymmetric PL band

378

covering 2.1-2.55 eV spectral range is seen under excitation of 2.69 eV. As the photoexcitation

379

energy is decreased, the PL band becomes narrower and is gradually shifted towards lower

380

energy side. There is a red-shift of PL band by 0.3 eV when the excitation energy is changed

381

from 2.69 eV to 2.38 eV. The red-shift indicates that the trapped charge carriers jump from

382

different defect states to recombine with the holes. Moreover, the asymmetric and broad PL band

383

attains

384

Broadening/narrowing of PL band and its asymmetric/symmetric characteristics depend on the

385

filling up of empty defect levels with photoexcited charge carriers. At higher excitation energy

narrow

and

symmetric

features

as

the

excitation

energy

is

decreased.

22


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386

the defect levels near the conduction band-edge are filled up with charge carriers (shown in

387

schematic in Figure 10b).

388 389 390 391 392 393

Figure 10 (a) Photoluminescence spectra of CN520 at different excitation energy. The excitations are below the optical bandgap of the grown nanosheets. (b) Schematic showing below bandgap excitation and emissions due to carrier trapped in different defect states. Different band states in the valence band and conduction band, nitrogen lone pair (LP) state and nitrogen-related defect states are shown. (c) TRPL spectrum (logarithmic plot) of CN520 excited at 2.45 eV.

394

395

When the initial defect levels are filled up, the remaining charge carriers hop to its underlying

396

defect levels. Inhomogeneous distribution of charge carriers in these states might have resulted in

397

a broad PL band at an excitation of 2.69 eV. The narrowing and symmetric feature of PL band at

398

lower excitation energy indicates homogenous carrier distribution in the underlying defect levels.

399

TRPL data is recorded for CN520 with excitation at 2.45 eV and monitored at the emission

400

energy of 2.24 eV (Figure 10c). The three components of carrier lifetime are very short in case of

401

defect emissions (Table S2). Unlike the case of free and bound excitonic emission having an

402

average lifetime of 17.4 ns and 27.4 ns, the average lifetime of defect emissions is very short of

403

the order of 0.14 ns. From the analyzed results of PL under above and below band gap excitation,

404

we might infer that nitrogen related defects could extend the lifetime of free exciton when we

405

choose the excitation energy above the bandgap. Below bandgap excitation releases free carriers, 23



Page 24 of 44

406

which are excited directly to the trapped centers underlying the bandgap of C3N4. The defect-

407

related PL band has low stability and decays with a very short lifetime.

4. Conclusions

408

409

In

summary,

we

employed

positron

annihilation,

steady-state

and

time-resolved

410

photoluminescence spectroscopy to probe the evolution of nitrogen-related defects in C3N4 with

411

calcination temperature. After photoexcitation, the excitonic emission in the intraplanar,

412

interplanar or intrachain direction gives rise to the shorter and longer decay components.

413

Excitons bound to nitrogen-related defect states decay with a lifetime of 27.4 ns, which is longer

414

than the free excitonic emission decay of 17.4 ns. Defect emission shows dependence on

415

excitation energy, and the lifetime of defect emission is only 0.14 ns. We have also demonstrated

416

laser power dependent excitonic emission in C3N4 with above bandgap excitation. The spectral

417

red-shift is mostly resulted by the band filling effect in which exciton readily fills up the

418

nitrogen-related defect states. This study provides a direct correlation between the defects

419

monitored by the PAS and PL and establishes the significant role of nitrogen-vacancy defects in

420

controlling the bound excitonic lifetime in C4N4 nanosheets. These results are significant for

421

tuning the optical properties of C3N4 nanosheets for ensuing applications in optoelectronics,

422

energy, and environment.

423

424

ASSOCIATED CONTENT

425

Figures of XPS, FTIR, TGA, Time-resolved PL, laser power dependent PL are included in the

426

Supporting Information. 24


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427

ACKNOWLEDGMENTS

428

The authors would like to thank central instruments facility (CIF) IIT Guwahati for providing

429

various characterisation facilities. BC would like to thank Department of Science and

430

Technology (DST), Govt. of India for providing DST Inspire faculty award (IFA15/MS-62) to

431

pursue this work. We thank Prof. M. Fujii, Kobe University, for the XPS measurement.

432 433

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Figure 1 XRD pattern of C3N4 synthesized at different calcination temperatures. Inset shows the expanded view of (002) peak revealing clear peak shift. 72x65mm (600 x 600 DPI)



Figure 2 Low- and high- resolution TEM images of: (a-b) CN420, (c-d) CN470, (e-f) CN520 and (g-h) CN570. 71x34mm (300 x 300 DPI)


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Figure 3 Core level (a) C 1s and (b) N 1s peak XPS spectrum of CN520 54x20mm (300 x 300 DPI)



Figure 4 (a) Ratio of area normalized CDB spectra of calcined C3N4 with respect to area normalized CDB spectra of 420 oC calcined C3N4. (b) Doppler broadening shape parameter, S-parameter, for differently calcined C3N4 samples. 53x20mm (300 x 300 DPI)


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Figure 5 (a) Kubelka-Munk absorption plots, and (b) Tauc plots for the band gap calculation for C3N4 samples calcined at different temperatures. 40x18mm (300 x 300 DPI)



Figure 6 (a) PL spectra of C3N4 nanosheets grown at different temperatures under an excitation energy of 3.35 eV. The inset shows a magnified view of the PL spectrum of CN420. (b) PL spectra of CN520 obtained under different excitation energies. 62x24mm (600 x 600 DPI)


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Figure 7 (a) Decovoluted (Gaussian) PL spectrum of CN520 under excitation of 3.35 eV. (b) TRPL (logarithmic plot) spectra of CN520 excited at 3.30 eV and monitored at 2.78 eV, 2.58 eV and 2.38 eV emission energies. (c) Plot showing a relation of S-parameter (left Y-axis) and average bound excitonic lifetime (tav at 2.38 eV) (right Y-axis) as a function of calcination temperature, which demonstrates a direct correlation between the S-parameter and tav. 80x42mm (600 x 600 DPI)



Figure 8 Schematic shows exciton migration in (a) π-stacking direction (interplanar), and (b) intraplanar migration (1) and intrachain (2) migration. Free excitons (F-exciton), when attached to nitrogen related defects (N-defects) create bound excitons (DB-exciton). Exciton migration is prohibited if C-H or N-H moiety terminates the heptazine link. 129x126mm (300 x 300 DPI)


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Figure 9 Normalized PL spectra of CN520 for different excitation power (intensity). The samples are excited with a solid-state laser of energy 3.06 eV. (b) Changes in emission energy (left Y-axis), full width at half maximum (FWHM) (right Y-axis) with laser power. The inset shows the PL emission intensity as a function of excitation laser power. 52x19mm (600 x 600 DPI)



Figure 10 (a) Photoluminescence spectra of CN520 at different excitation energy. The excitations are below the optical bandgap of the grown nanosheets. (b) Schematic showing below bandgap excitation and emissions due to carrier trapped in different defect states. Different band states in the valence band and conduction band, nitrogen lone pair (LP) state and nitrogen-related defect states are shown. (c) TRPL spectrum (logarithmic plot) of CN520 excited at 2.45 eV. 60x20mm (300 x 300 DPI)


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Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride

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