cu3p is 
 a copper - rich alloy that is commonly used in soldering , and it has 
 been additionally investigated as anode material in lithium ion batteries . some reports indicate that cu3p is a semiconductor ( with 
 a gap of 1.31.4 ev according to scanning tunneling spectroscopy or around 0.8 ev according to optical spectroscopy ) , while other works have shown that this material 
 has a behavior closer to that of a metal . a 
 study by olofsson in 1972 on the cu p alloy system demonstrated 
 that the homogeneity range lies between cu2.867p and cu2.755p . upon detailed crystal structure 
 analysis , it emerged that the deviation from the 3:1 cu : p stoichiometry 
 was associated with cu vacancies on the two 6(c ) copper positions 
 in the cell . our group has recently developed 
 synthetic approaches to cu3p nanocrystals ( ncs ) in shapes 
 that depended on the type of the synthesis ( either platelets or truncated 
 hexagonal bipyramids ) . soon after that , 
 pradhan et al . reported a detailed study on platelet - shaped cu3p ncs in which the optical absorption 
 band in the 13 m region of the spectrum was interpreted 
 as due to a localized surface plasmon resonance ( lspr ) . the claim 
 was supported by the dependence of the spectral position of the absorption 
 band on the refractive index of the solvent in which the particles 
 were dissolved . it is likely that such plasmonic behavior arises from 
 the presence of copper vacancies ( as described by olofsson ) , a finding that would go hand in hand with recent 
 developments on plasmonic copper chalcogenide ncs , in which the lspr 
 can arise from a high density of free holes in the valence band , due 
 to the many cu vacancies that these structures can sustain . we have carried out density functional calculations on the 
 formation energy of cu vacancies in cu3p . the results of 
 our calculations indicate indeed that the formation energy of cu vacancies 
 in 6(c ) copper positions is negative , and therefore they are thermodynamically 
 favored ( in line with the findings of olofsson ) , and henceforth this enables us to refer to the cu3p ncs , with which we will deal in this paper , as having a cu3-xp  stoichiometry . also , hot - probe 
 thermoelectric measurements on films of platelet - shaped cu3-xp ncs ( synthesized by us ) pointed to a metallic p - type 
 behavior for such ncs . we then studied the time - dependent evolution 
 of the optical absorption of cu3-xp ncs in the nir , which was supportive of a lspr , as first reported 
 by pradhan et al . our studies demonstrate 
 that most likely both the p - type metallic character and the presence 
 of a lspr in the ncs stem from the thermodynamically favored substoichiometry 
 in cu . these findings make cu3-xp ncs a new and alternative promising material if compared to copper 
 chalcogenide ncs . furthermore , inspired by the recently discovered 
 ability of cadmium phosphide ncs to undergo 
 cation exchange with a variety of cations ( with preservation of nc 
 size , shape , and often of the anion framework ) we explored the feasibility 
 of such reactions on cu3-xp ncs . this is further supported by the possibility to partially displace 
 cu with li ions in cu3p ( leading 
 to cu3-zlizp ) via electrochemical 
 lithiation . we demonstrate here that the 
 hexagonal anion framework of cu3-xp can accommodate for cu cations to be displaced by in cations without considerable distortions and allows for 
 the formation of metastable wurtzite ( wz ) inp ncs even at mild reaction 
 conditions . the exchange reaction starts on the peripheral corners 
 of each nc and proceeds toward the center forming as intermediate 
 steps multidomain cu3-xp / inp heterostructures . sham density functional theory ( dft ) using the pseudopotential approximation and the pbe functional . more details on 
 the calculations can be found in the supporting 
 information ( si ) . copper(i ) chloride ( cucl , 
 99.999% ) , trioctylphosphine oxide ( topo , 99% ) , and tri - n - octylphosphine 
 ( top , min . indium(iii ) bromide 
 ( inbr3 , 99.999% ) , oleylamine ( olam , 70% ) , octylamine ( octam , 
 99% ) , tetrachloroethylene ( tce , anhydrous , 99% ) , tri - n - butylphosphine ( tbp , 97% ) , and 1-octadecene ( ode , 90% ) 
 were purchased from sigma - aldrich . anhydrous chloroform , toluene , 
 and ethanol absolute were purchased from carlo erba reagents . the 
 synthesis of cu3-xp ncs was carried 
 out following our previous work with minor modifications . in a typical synthesis , a solution of cucl ( 3.2 
 mmol ) , degassed olam ( 4.8 ml ) , and degassed octam ( 0.8 ml ) was prepared 
 under inert atmosphere and then heated to 140 c for 1 h to get 
 a clear solution . a mixture of 8 g of topo and 12 ml of top was degassed 
 in a reaction flask for 2 h at 130 c under vacuum using a standard 
 schlenk line . the copper solution was then then rapidly injected into 
 the reaction flask at 380 c . after the injection the temperature 
 dropped , and the solution was heated to 350 c for 50 min and 
 then slowly cooled to room temperature . the cu3-xp ncs were washed twice by dispersion in toluene 
 followed by precipitation by addition of ethanol . extra olam ( 100 
 l ) was added at each cleaning step to optimize the colloidal 
 stability of the ncs . eventually the cu3-xp ncs were dispersed in toluene and stored in a glovebox under 
 inert atmosphere . in order to exchange cu with in ions we followed the method reported by beberwyck et al . with various modifications . in a typical cation 
 exchange reaction , a solution of inbr3 ( 0.2 mmol ) , top 
 ( 1.5 ml ) , and ode ( 3.5 ml ) was degassed at 130 c for 1 h and 
 then heated to 200 c . at this point a solution of cu3-xp ncs in degassed ode ( 1 ml ) , containing 0.2 mmol 
 of cu atoms ( that is a in : cu molar ratio of 1:1 ) , was added to the 
 reaction flask , and the solution was allowed to react for 15 min at 
 200 c . the ncs were washed twice by dissolution in chloroform 
 followed by precipitation upon addition of ethanol . in the first cleaning step a small amount of degassed octam ( 1 ml ) was added in order to 
 better stabilize the colloidal suspension . the samples were 
 prepared by dropping dilute solutions of ncs onto carbon coated gold 
 grids that were then placed in a pumping station in order to let the 
 solvent evaporate completely and preserve them from oxidation . low 
 resolution transmission electron microscopy ( tem ) measurements were 
 carried out on a jeol-1100 transmission electron microscope operating 
 at an acceleration voltage of 100 kv . high resolution tem ( hrtem ) 
 was performed on a jeol jem-2200fs microscope equipped with a schottky 
 emitter at 200 kv and an in - column omega filter . the chemical composition 
 of the ncs was determined by energy dispersive x - ray spectroscopy 
 ( edxs ) analysis performed in high angle annular dark field scanning 
 tem mode ( haadf - stem ) with a jeol jed-2300 t detector , using the cliff - lorimer 
 method . xrd measurements were performed 
 on a rigaku smartlab x - ray diffractometer operating at 40 kv and 150 
 ma . the diffractometer was equipped with a cu source and a gobel mirror 
 in order to have a parallel beam , and it was used in the 2-theta / omega 
 scan geometry for the acquisition of the data . specimens for the xrd 
 measurements were prepared in the glovebox by dropping a concentrated 
 ncs solution onto a zero - background silicon substrate . this was carried out via inductively coupled plasma 
 optical emission spectroscopy ( icp - oes ) , using a icap 6500 thermo 
 spectrometer . all chemical analyses performed by icp - aes were affected by a systematic error of about 5% . raman measurements were performed under inert 
 atmosphere by fluxing nitrogen through a closed chamber ( by linkam ) 
 in order to avoid oxidation of the samples under laser irradiation . raman spectra were collected using a renishaw invia microraman spectrometer 
 that excites the samples with a diode laser (  = 785 nm ) focused 
 through a 50 magnification microscope objective at a nominal 
 power of 200 mw and integration times up to 60 s. samples for xps were prepared under inert 
 atmosphere by dropcasting a solution of ncs onto a highly oriented 
 pyrolytic graphite ( hopg , zyb grade , nt - mdt ) substrate . sample preparation 
 was performed in a n2-filled glovebox , and the specimens 
 were then transferred into the xps setup via a transfer vessel to 
 avoid exposure to air . measurements were performed on a kratos axis 
 ultra dld spectrometer , using a monochromatic al k source ( 15 
 kv , 20 ma ) . high resolution narrow scans were performed at constant 
 pass energy of 10 ev and steps of 0.10 ev . the photoelectrons were 
 detected at a takeoff angle of  = 0 with respect to the 
 surface normal . the pressure in the analysis chamber was maintained 
 below 7  10 torr for data acquisition . the 
 data was converted to vamas format and processed using casaxps software , 
 version 2.3.16 . the binding energy ( be ) scale was internally referenced 
 to the c 1s peak ( be for c cu3-xp ncs 
 stabilized with olam and dissolved in toluene were dropcast on microscope 
 glass slides and allowed to dry in a glovebox under inert atmosphere . 
 to improve the conductivity of the films , the samples were annealed 
 on a hot plate at 200 c for 30 min under nitrogen atmosphere . this annealing process promotes a partial removal of the residual 
 organic in excess and results in a significant improvement of the 
 film conductivity . subsequently , ohmic contact pads ( 2 mm ) were deposited on the films by sputtering 20 nm of au ( 99.99% ) 
 in an ar - filled chamber ( cressington sputter coater 208hr ) with a 
 metal shadow mask tightly mounted on the top of the sample ( see figure s4 for the linear current the samples 
 were mounted on a copper heat sink to facilitate the heat dissipation 
 from the substrate . the ncs ( dispersed in toluene ) were dried under nitrogen flux to 
 remove the solvent , and they were then redispersed in anhydrous tce 
 to perform the measurements . the uv  vis - nir absorption spectra 
 of the ncs solutions were recorded using a varian cary 5000 uv  vis - nir 
 absorption spectrophotometer . probe 
 measurements was based on a ti : sapphire chirp pulse amplified source , 
 with maximum output energy of about 1 mj , 1 khz repetition rate , central 
 wavelength of 800 nm ( 1.59 ev ) , and pulse duration of about 180 fs . excitation pulses at 1300 nm ( 0.95 ev ) were generated by noncollinear 
 optical parametric amplification ( nopa ) in a -barium borate 
 ( bbo ) crystal , with pulse duration of around 150 fs . probing was done in the near 
 ir region ( 870 nm1500 nm ) by using white light generated in 
 a thin sapphire plate . chirp - free transient transmission spectra were 
 collected by using a fast optical multichannel analyzer ( oma ) with 
 dechirping algorithm . we used a ingaas bayspec supergamut nir spectrometer 
 with a spectral range from 870 to 1650 nm . excitation energy per pulse has been kept in the linear 
 regime at around 450 nj ( measured at 1300 nm ) , in order to get 360 
 j / cm fluence . all measurements were performed at 
 room temperature on sealed samples prepared under nitrogen atmosphere 
 by dispersing cu3-xp ncs ( containing 
 0.005 mmol of cu ) in 0.7 ml of toluene . the cu3-xp ncs resulting from our synthetic route are typically 
 polydispersed hexagonal shaped platelets with a diameter ranging from 
 10 nm to 50 nm . higher reaction times ( i.e. , more 
 than 50 min ) result in larger ncs that can be grown as large as 100 
 nm , albeit with a broader size distribution . the xps analysis of the as - synthesized cu3-xp ncs is consistent with the presence of cu and 
 p species ( giving a rough estimation of the cu : p 
 ratio of 2.9:1 ) and excludes any trace of cu ions ( see 
 the si and figure s7 for further details ) . according to their xrd pattern ( reported later in this work in figure 4c and in figure s8 ) , 
 the ncs crystallize in a hexagonal phase ( space group p63 cm , see figure 1a - b ) . refinement 
 of the structural parameters using the fundamental parameter ( fp ) 
 method yielded the following lattice 
 parameters : a = 6.9507  and c = 7.1428  ( see figure s8 ) that 
 are consistent with a cu3p structure substoichiometric 
 in copper , as shown by olofsson . the asymmetric unit of cu3p is composed of one p atom 
 and four cu atoms . along the   direction , the different irreducible 
 cu positions can be grouped in adjacent layers ( see figure 1a ) . p atoms are arranged in a hexagonal sublattice , 
 which is intercalated between the cu3 and cu4 layers . the resulting 
 structure can be described in terms of small aggregated subunits composed 
 of one p atom coordinated with 11 cu sites , with an average p the structural anisotropy 
 leads to partial ionic bonding in the compound , as evidenced by our 
 atom - in - molecule analysis ( see the computational details section in 
 the si ) . according to the bader partitioning 
 scheme , charge transfer occurs in cu3p , leading to a charge 
 depletion of 0.2 e on each cu atom and a charge 
 excess of 0.6 e on p atoms . these values are smaller 
 than the respective formal charges , consistent with the mixed ionic / covalent 
 character of the compound . the calculated lattice parameters for the 
 stoichiometric cu3p cell were a = 7.04 
  and c = 7.32  . these parameters are 
 slightly larger than the bulk experimental values ( a = 6.96  , c = 7.14  ) which are indeed 
 very close to the parameters extrapolated for our cu3-xp ncs ( a = 6.95 and c = 7.14 , see previous section ) . a slight overestimation of lattice 
 parameters is expected for pbe calculations on solids . in the systems with one vacancy ( either 
 in cu1 , cu2 , cu3 , or cu4 sites ) the cell contracted slightly along 
 the c axis ( optimized lattice parameters were a = 7.04  , and c = 7.18  ) . we computed the values of the single vacancy formation energy ev(1 ) ( see the si for details ) , in either cu1 , cu2 , cu3 , or cu4 sites . the 
 vacancy formation energy has been estimated in the range of values 
 for the cu chemical potential , cu , compatible with the thermodynamic stability of cu3p 
 ( see also the si for a more in - depth explanation ) . 
 the higher and lower bounds of cu correspond , respectively , to the cu - rich condition , i.e. cu = cu(bulk ) , and p - rich condition , i.e. p = p(bulk ) , where cu(bulk ) and p(bulk ) represent the chemical potentials of the elemental bulk cu 
 and p. for values of cu larger 
 than cu(bulk ) , cu3p 
 is unstable and copper precipitates into the pure elemental solid . similarly , for values of cu smaller 
 than the lower bound , cu3p is unstable and elemental solid 
 phosphorus forms . the vacancy formation energies ev(1 ) are negative in cu1 ( 117 
 mev ) and cu2 ( 99 mev ) sites , while they are positive in cu3 
 ( 162 mev ) and cu4 ( 54 mev ) sites . ( c ) free energy profile for ev(2 ) at the v2 site , as a function of 
 cu . the line y = 0 
 is also shown to help distinguish regions in the free energy profile 
 where 1 or 2 vacancies are energetically favored . from these calculations we can conclude that even in copper 
 rich conditions at least one vacancy would form , preferably in cu1 
 sites , which corroborates the substoichiometric nature of the material 
 proposed by olofsson . starting from the 
 lowest energy system with one single vacancy , we introduced one additional 
 vacancy in one of the remaining 17 nonequivalent cu sites . cell optimization 
 led again to a small decrease of the lattice parameters ( a = 7.02  , c = 7.16  ) in the lowest energy 
 structure . the formation energies of the second vacancy ev(2 ) , in cu - rich conditions are shown 
 in table s1 of the si . the stability of the second vacancy 
 can be increased by moving to p - rich conditions . this can be seen 
 in figure 1c , where we show that the formation 
 energy of the second vacancy becomes negative in the p - rich corner . the most stable vacancy pair found in our dft calculations is therefore 
 the one corresponding to the unoccupied sites cu1 and cu2 , denoted 
 as v1 and v2 and described in figure 1a - b . these results are again in close agreement with the crystallographic 
 refinement presented by olofsson , where 
 it was indicated that cu1 and cu2 are below full occupancy . the computed 
 values for ev(3 ) are 
 instead positive over the whole stability range of cu3p , 
 suggesting that the presence of one or two vacancies per cell is the 
 most likely situation in cu3-xp . this set of results confirms the scenario that cu3p 
 is natively substoichiometric and that its composition varies from 
 16 cu and 6 p atoms per unit cell ( cu2.667p ) in p - rich 
 conditions to 17 cu and 6 p atoms ( cu2.833p ) in cu - rich 
 conditions . another interesting aspect of consistence between 
 our calculations and the work of olofsson is represented by the experimental sharp change in the lattice parameter 
 upon increase of the compositional p atomic percentage . this structural 
 transition is explained by our calculations , which reproduce a quantitatively 
 similar change in the lattice parameter upon increasing the number 
 of copper vacancies from one to two . because of the large number 
 of possible cu vacant sites in cu3p , we expect that the 
 configurational entropy strongly favors the increase of vacancies 
 in the system with temperature . consistent with this view , the statistical 
 model presented in the si suggests that 
 the formation of vacancies in the cu - rich side of the stability diagram 
 is strongly activated by temperature . at t = 500 
 k for instance the content of cu vacancies at cu = cu  ( bulk ) increases by about 20% with respect to the t = 
 0 k case ( see the black curve in figure s2 ) , within the level of modeling used . the marked dependence of the 
 total number of vacancies on temperature correlates with the necessity 
 of working at about 500 k in the cu3p - inp cation exchange 
 experiments ( see later ) . hot - probe thermoelectric measurements 
 on cu3-xp nc films were carried 
 out to determine the majority carriers in our ncs . the setup of the 
 measurements is illustrated in figure 2a . in 
 order to ensure that no alteration of the ncs occurred in the annealing 
 process that preceded the electrical measurements , xrd patterns and 
 xps analyses of the nc films were acquired before and after the heat 
 treatment ( see figures s5 and s7 ) . in the 
 thermoelectric measurements , the voltage difference between probe 
 1 and probe 2 was recorded over a period of time ( typically 100 
 s ) by a voltmeter ( agilent 34410a ) with its positive terminal connected 
 to the hot probe and the negative terminal to the room temperature 
 ( rt ) probe . a soldering iron was used as a heating source . in the 
 first few seconds of each measurement the sample was not heated up 
 yet . then the iron was carefully brought in contact with the glass 
 substrate and heated up to 380 c . being the iron closer to probe 
 1 , it created a temperature gradient between probe 1 and probe 2 . after 50 s the heating iron was quickly removed from the substrate 
 surface , allowing the sample to cool down to room temperature . in the thermoelectric measurements , the majority carrier type of 
 the film can be identified from the polarity of the thermoelectric 
 voltage that manifests between the cold and the hot probes . for the cu3-xp nc films a positive space charge builds up at the cold probe ( leading 
 to a negative voltage drop as shown in figure 2b ) , which can be rationalized by a diffusion of positive majority 
 carriers from the hot to the cold probe , thus pointing to p - type conduction 
 ( see also control experiments in figures s6 on si wafers with known conductivity type ) . when the heating iron 
 was removed from the substrate at t = 57 s , the thermoelectric 
 voltage dropped quickly back to zero , suggesting efficient heat dissipation 
 from the sample to the surrounding environment . ( a ) sketch illustrating 
 the thermoelectric measurement setup on a cu3-xp nc film . an iron soldering tip is in contact with the glass 
 substrate in close proximity to probe 1 on the nc film . the voltage 
 drop across the two gold pads is measured by means of a voltmeter . the circled positive signs represent the holes diffusing from the 
 hot probe to the probe at rt . ( b ) thermovoltage across the gold pads 
 measured over time while heating the soldering iron from rt up to 
 380 c ( red area ) and then quickly removing it from the glass 
 substrate ( light blue area ) . the results of the thermoelectric measurements indicate that 
 our cu3-xp ncs behave as semiconductors 
 and that holes are the majority carriers . we believe that their conductivity 
 arises from the large number of cu vacancies in the cu3-xp lattice ( at least one per unit cell ) that create 
 a large number of acceptor levels close to the valence band . on the 
 other hand , our dft calculations point to a ground state of cu3p that is semimetallic for both stoichiometric and substoichiometric 
 systems , although the presence of cu vacancies affects the dft electronic 
 structure by enhancing the energy difference between the highest occupied 
 and lowest unoccupied band of the system ( see the si for more details ) . the experimental semiconducting state 
 may be obtained by the use of quasi - particle corrections to the standard 
 dft results and/or after introducing structural disorder in the system . similar considerations have been used to describe the 2-fold metallic / semiconducting 
 character observed in some copper chalcogenides , such as cus and cu2s . as mentioned in the introduction , previous 
 works supported the presence of lspr in cu3p ncs , which 
 should be elicited by free carriers ( holes ) . for a further verification of the plasmonic nature of the nir resonance 
 that is also characterizing our cu3-xp ncs ( see figure 3a ) , we performed 
 transient absorption ( ta ) measurements in the region of the nir resonance . we pumped at 1300 nm by generating pulses of around 150 fs through 
 a nopa in a bbo crystal , which is close to the maximum nir resonance , 
 and we probed in the nir , around 8701500 nm , by using a white 
 light generated in a thin sapphire plate ( see figure 3a ) . a nonlinearity was observed in the region of the nir resonance . figure 3b displays a contour plot showing the 
 wavelength ( y - axis ) versus time ( x - axis ) of the differential transmission ( t / t % ) . as demonstrated also in the time trace in 
 figure 3c , an instantaneous bleach signal was 
 observed at 1500 nm . the bleach maximum , at around 1500 nm , was very 
 close to the steady state nir absorption maximum . ( a ) steady - state absorption 
 of the cu3-xp ncs ; the arrow indicates 
 the pump wavelength while the probe region is marked in green . ( b ) 
 contour plot of the differential transmission versus wavelength ( y - axis ) and time ( x - axis ) . horizontal dashed 
 lines in black , blue , and red indicate the wavelength of representative 
 differential transmission decay dynamics as given in ( c ) at 1500 nm 
 ( black curve ) together with a biexponential fit ( red curve , upper 
 plot ) , and 900 nm ( red curve , lower plot ) and ( d ) at 1100 nm ( blue 
 curve ) together with a fit to the oscillation as described in the 
 main text ( orange curve ) . the observed nonlinearity was dominated by a two - step decay , 
 having a fast initial component with a time constant of around 1 ps 
 and a second slower decay of around 50 ps ( the biexponential fit is 
 given in figure 3c ( red curve ) upper panel 
 together with the 1500 nm decay in black ) . to the blue end of the 
 spectrum this negative 
 feature is represented by the decay dynamics at 900 nm ( red curve 
 in figure 3c , lower panel ) . it showed a similar 
 temporal behavior as the bleach maximum . we interpreted the bleach 
 and the photoinduced absorption signals as an excitation - induced damping 
 and broadening of the plasmonic resonance due to the pump pulse that 
 induced heating of the carrier gas . this in turn led to an induced 
 absorption in the transient spectra to the red and the blue end of 
 the plasmon resonance , and a decrease of intensity around the plasmon 
 maximum , detected as a bleach signal in the ta measurements . while we observed the bleach signal close to the lspr maximum and 
 the photoinduced absorption to the blue end , between 870 and 
 1200 nm , an evaluation to the red was not possible , due to a limited 
 white light generation in this spectral region . the observed 
 time - dependent evolution of the optical spectra in the region of the 
 nir resonance is clearly reminiscent of the typical two step decay 
 dynamics observed in plasmonic nanostructures , and it can be understood in the 
 following way : the initial very intense pump pulse excites the free 
 carriers in the system to a nonthermal energy distribution , which 
 then thermalize within the duration of the laser pulse . this leads 
 to a fermi distribution at a higher carrier temperature , which results 
 in the maximum of the ta signal observed . thereafter , the carriers 
 relax within a couple of picoseconds by releasing their energy through 
 the emission of phonons , leaving the lattice at an elevated temperature . afterward , the lattice cools down with a slower decay through 
 the interaction with the surrounding medium until it reaches its equilibrium 
 after a couple of hundreds of picoseconds . in the dynamic trace at 1100 
 nm ( blue curve in figure 3d ) oscillations are 
 found up to 20 ps . to extract the period of the oscillation we fitted 
 the dynamics at several wavelengths , starting from delays greater 
 than 1.5 ps and by previously subtracting the slowly varying second 
 exponential decay . then the period t and the damping 
  were extracted by using a damped cosine function , which describes 
 the oscillatory behavior1where a and  are free 
 parameters to obtain a valuable fit ( figure 3d , orange curve ) . the extracted values are t = 6.936 
 ps and  = 3.825 ps . for plasmonic ncs the oscillatory behavior 
 of the ta signal is related to an instantaneous increase in the lattice 
 temperature due to the carrier phonon interaction , which in turn causes 
 a small expansion of each single nc . as a consequence of the activation 
 of its vibrational modes an oscillating and periodic change of its 
 dimension , shape , and volume is induced . as the plasmonic resonance depends both on the particle geometry 
 and the density of the carriers , a modulated absorption of the pumped 
 ncs is detected . importantly , our results demonstrate that our ncs contain a sufficient 
 concentration of free carriers to absorb the energy of the high intensity 
 pump pulse and to transfer it to the lattice efficiently leading to 
 the coherent excitation of the breathing modes in the ncs . similar 
 results have been found in the recently examined copper chalcogenide 
 ncs , where a high hole density was responsible for the observed nonlinearity . our ta results thus show the typical time dependent spectral behavior 
 characteristic of plasmonic nanostructures and confirm the plasmonic 
 nature of the nir resonance in cu3-xp ncs . the partial ionic character of cu3-xp , as calculated and discussed earlier , suggested that the 
 replacement of the cu(i ) cation sublattice in this material should 
 be feasible . we developed a procedure to exchange cu ions 
 with in ions in cu3-xp ncs , by adapting a procedure reported by beberwyck et al . ncs , the exchange occurred at 
 conditions that were milder than those tested on cd3p2 ncs ( typically carried out at 300 c and with large 
 excess of incoming ions with respect to cd ions ) . for example , a total conversion of cu3-xp to inp ncs could be achieved at temperatures as 
 low as 200 c and with a molar ratio of in precursor 
 to cu ions present in the ncs as low as 1:1 ( that is , 
 three times the required stoichiometric ratio ) . figure 4 reports xrd , raman , and tem data on initial , 
 intermediate , and final samples of an exchange reaction ( tem images 
 refer only to the initial and final samples ) ( see the si for more details on partial exchange experiments ) . low resolution 
 tem images of hexagonal platelet - shaped ( a ) cu3-xp and ( b ) inp ncs after cation exchange . ( c ) xrd patterns and ( d ) raman spectra 
 obtained from dropcast solutions of cu3-xp , cu3-xp / inp , and inp ncs . 
 in ( c ) the bulk reflections of cu3-xp ( icsd card no . 15056 ) and wz inp , the latter calculated from the 
 icsd card no . ( d ) , labeled as cu3-xp / inp and 
 inp , it can be noticed the presence of transverse optical ( to , at 
 304 cm ) and longitudinal optical ( lo , at 343 cm ) phonon first order modes and of the lo phonon second 
 order mode at 687 cm of inp , in accordance with 
 literature data . at a deeper analysis , it is possible to note that the lo peak , even if characterized by 
 a narrow profile ( fwhm = 5 cm ) , shows an asymmetric 
 broadening at lower frequencies , associated with contributions from 
 the surface modes of the crystals . ( e ) 
 uv  vis - nir absorption curves of solutions of cu3-xp , cu3-xp / inp , 
 and inp ncs dispersed in tce . according to tem , both size and morphology of the cu3-xp ncs were retained , even after 
 complete exchange ( compare panels a and b in figure 4 ) . xrd patterns of the initial , intermediate , and final samples 
 are reported in figure 4c . an evolution can 
 be evinced from the hexagonal cu3-xp phase ( icsd card no . 15056 ) of the initial sample ( figure 4c , bottom pattern in black ) to a phase that could 
 be indexed to wz inp ( icsd card no . 180911 ) for the fully exchanged 
 sample ( figure 4c , top pattern in red ) , passing 
 through an intermediate sample with a mixed pattern of cu3-xp and wz inp phases ( figure 4c , middle pattern in blue ) , with no additional phases present . as 
 a note , in order to have a good fit of the experimental data for wz 
 inp , we had to refine the structural parameters ( using the fp method ) , 
 which yielded a = 4.1420  and c = 6.8095  . therefore , the wz inp phase of our exchanged inp 
 ncs was characterized by slight compression along both the ab plane ( 0.2% ) and the c direction ( 1.5% ) 
 with respect to the reference wz inp phase from the database . here , as in a previous work of ours , raman analysis turned out to be a valid tool 
 to assess whether alloyed crystals or heterostructures were formed 
 in the intermediate steps . raman analysis of the same initial , intermediate , 
 and final sample ( reported in figure 4d ) supported 
 a cu3p inp transformation , upon exchange with in ions , with no presence of alloys nor other compounds other 
 than cu3p and inp at the intermediate step : while the pristine 
 cu3-xp ncs ( figure 4d , bottom spectrum in black ) did not exhibit any raman feature , 
 both the intermediate ( figure 4d , middle spectrum 
 in blue ) and the final ( figure 4d , top spectrum 
 in red ) spectra were characteristic of the inp phase ( see caption 
 of figure 4 for more details ) . further insights over the cation exchange process 
 could be deduced from the optical absorption spectra of solutions 
 containing initial cu3-xp ncs , 
 the intermediate sample , and the final inp ncs , respectively . as shown 
 in figure 4e , the initial copper phosphide 
 ncs ( black curve ) a weaker nir absorption in the intermediate 
 sample ( blue curve in figure 4e ) , with a maximum 
 still at about 1500 nm , most likely arose from the residual cu3-xp domains in the partially exchanged 
 heterostructures . the final inp ncs ( red curve ) did not show any feature 
 in the nir region and exhibited instead an absorption edge having 
 its maximum at 800 nm from which we extrapolated a bandgap 
 of 1.55 ev . this value is slightly larger than the bulk band gap of 
 wz inp ( 832 nm , 1.49 ev ) , thus we believe that a quantum confinement of carriers might take 
 place in our inp ncs . since the bohr radius of bulk zinc - blende inp 
 is around 110  ( 11 nm ) , the confinement 
 in our inp nanoplatelets should be along their ( 001 ) direction ( the 
 one perpendicular to the basal facets ) . this is reasonable , as the 
 thickness of some ncs could be as low as 5 nm ( see figure 6b ) , while the lateral dimensions were much larger . the intermediate sample was a mixture of totally exchanged 
 wz inp nanocrystals and of partially exchanged cu3-xp / inp heterostructures . typical haadf - stem images 
 of partially exchanged cu3-xp ncs 
 ( figure 5a - c ) evidence the presence of two 
 different types of domains inside each nc . the brighter domains are 
 cu3-xp , while the darker ones are 
 inp ( verified from edxs maps ) . from the sequence of haadf images , 
 which capture three ncs at different stages of conversion , it can 
 be seen that the exchange starts preferentially from some of the corners 
 of the hexagonal platelets ( figure 5a ) . one 
 possible explanation of this side selectivity would be that , while 
 the upper and lower facets of each cu3-xp platelet are almost atomically flat ( see also our previous 
 work on cu3-xp ncs ) , the lateral edges and especially the corners contain 
 more low - coordination species , that is highly reactive sites . in particular , 
 in the direction of the six corners of the platelets ( i.e. , [ 110 ] 
 and the other five equivalent directions ) straight cu atomic columns 
 can form preferential avenues for the inflow / outflow of cations ( see 
 also the sketches in figure 6 ) with a small 
 perturbation of the anion sublattice . this can explain why a preferential 
 substitution is seen starting from the sides of the ncs rather than 
 from the top / bottom . the exchange then progressed to yield heterostructures 
 that were often of the  pie  type reported in figure 5b . ( a - c ) haadf - stem of cu3-xp / inp heterostructures at different cation exchange states . ( d - e ) hrtem images at different 
 magnifications of the heterostructure shown in panel ( b ) , exhibiting 
 two different crystalline domains : cu3-xp in the two central wedges and inp in the upper and lower 
 wedges of the platelet . ( f ) detail of an interface between a cu3-xp and an inp domain in the heterostructure , 
 clearly showing the preservation of the anions sublattice . the inp wz cell is obtained from a 30 rotation of 
 the cu3-xp hexagonal cell , so that 
 cu3-xp ( 120 ) planes become 
 inp ( 110 ) planes ( following the notation used in figure 6c ) . ( h ) schematic representation of the cation exchange 
 reaction involving cu3-xp ncs and 
 in ions . hrtem images of a typical pie - shaped cu3-xp / inp nc are displayed in figure 5d - f . this heterostructure , with quite sharp cu3-xp / inp interfaces ( figure 5f ) , further supports our xrd and raman analysis that no other alloys / compounds 
 seem to be formed during exchange . also , from the fft of the hrtem 
 images of partially exchanged cu3-xp ncs ( see for example figure 5 g and s10 ) a clear 30 rotation relationship between 
 the cu3-xp and inp unit cells can 
 be extrapolated , from which we can infer the epitaxial relation between 
 the two structures : ( 100 ) inp || ( 210 ) cu3-xp and ( 001 ) inp || ( 001 ) cu3-xp . in this way the anions sublattice ( brighter columns in the intensity image of figure 5f ) is preserved at the interface . the formation of cu3-xp / inp nc heterostructures is well 
 supported by the experimental evidence that in the bulk there are 
 no known stable cu  in  p alloys , and even if cu is a 
 typical dopant for inp , attempts to diffuse 
 cu into bulk inp above doping levels ( typically smaller than 5  
 10 cm ) resulted in the formation 
 of cu  in metallic precipitates in inp . at later 
 stages of exchange , the ncs had domains that were often of the type 
 reported in figure 5c ( the nonexchanged cu3-xp domains were further confined 
 toward the center of the crystal or at domain boundaries ) , after which 
 complete cation exchange was accomplished ( see also the scheme in 
 figure 5h ) . in the fully exchanged samples , 
 hrtem analysis revealed that the cation exchange process led to the 
 formation of monocrystalline inp ncs exhibiting the expected wz structure , 
 as shown in figure 6a - b , 
 with no detectable defects . the lattice parameters extrapolated from 
 the hrtem images ( a = b = 4.21  
 and c = 6.80  ) were in good agreement with 
 the values calculated for the wz inp phase . edxs elemental analysis 
 performed on such inp ncs gave typical elemental compositions of the 
 following : p = 42% , in = 57% , and cu = 1% ( see also figure s9 of the si ) . these are in agreement with icp elemental 
 analysis , which corroborated the complete exchange of copper with 
 indium , with a residual amount of cu in the samples that did not exceed 
 2% . figure 6c reports atomic sketches 
 representing the [ 110 ] ,   lattice slabs of hexagonal cu3-xp and ,   lattice slabs 
 of wz inp . the similarity in the p anion frameworks of the two structures 
 is indeed noteworthy : as a consequence of replacement of cu ions with in ions , the initial cu3-xp lattice undergoes an expansion of 3% along 
 the ab plane ( acu3-xp = 6.95  and 3ainp = 7.17  ) and a contraction of 5% along 
 the c direction ( ccu3-xp = 7.14  and cinp = 6.81  ) . such a close match in anion sublattices 
 for the two structures may help to explain why the p anion sublattice remained unaltered during the whole process , and 
 single domain crystalline inp ncs were obtained at the end . it is known that cu ions can rapidly diffuse in bulk 
 inp ( both bulk and nanocrystalline ) through interstitial sites allowing for cu doping of inp ncs even at very 
 low temperatures ( i.e. , 50 c ) . on 
 the other hand , it is not clear whether in diffusion 
 in cu3-xp proceeds likewise interstitially 
 or it is instead dominated by cu vacancy diffusion . we believe that 
 the diffusion of in in the cu3-xp phase is most likely mediated by cu vacancies and that a 
  minimum  vacancy generation rate in cu3-xp ncs is required for the effective in diffusion . this can be achieved by increasing the reaction temperature , 
 as we estimated in our calculations ( details are reported in the si ) . at room temperature , c the mean value increases 
 to 1.2 , that is , there are 20% more vacancies available for 
 the ions diffusion ( see the black curve in figure 
 s2 ) . thus , it is likely that the limiting step of the exchange 
 process in our system is the in diffusion inside the 
 cu3-xp lattice that is directly 
 connected to the amount of cu - vacancies . clearly , higher reaction 
 temperatures ( with respect to room temperature , at which no cation 
 exchange was observed ) can trigger other processes that are required 
 for the exchange to take place , for example a more frequent detachment 
 of the ligand molecules at the surface of the ncs , allowing for the 
 insertion and the removal of ions . ( a - b ) hrtem images and the corresponding 
 fft ( of platelet - shaped inp ncs ) . the wz structure can be directly 
 inferred from the top ( a ) and the side ( b ) views . ( c ) atomic sketches 
 representing ( left panel ) [ 110 ] and   lattice slabs of 
 hexagonal cu3-xp and ( right panel ) 
 ,   lattice slabs of wz inp . the structural isomorphism of 
 the two phases is evident , with preservation of the anion sublattice . the projection of the primitive unit cell is depicted with a solid 
 black line in both structures . additional control experiments were run in order to establish 
 the best reaction conditions for the in  cu exchange reaction . interestingly when tbp was employed instead 
 of top , at the same reaction conditions , the resulting ncs exhibited 
 a very low inp conversion ( see the si ) . 
 while tbp has been shown to be a good soft base for the transformation 
 of ionic cux ( x = s , se , te ) into ii this can be explained considering that the cu3p  
 inp conversion requires a larger driving force in order to take place 
 and optimized reaction conditions are necessary . these experiments 
 prove that the soft acidity of cu ions is best matched 
 by the basicity of top rather than tbp . finally , worthy of note 
 is that , up to now the metastable wz inp phase has been observed almost only in nanowires grown by molecular beam 
 epitaxy or by chemical vapor deposition methods with the vapor  liquid  solid 
 ( vls ) technique . the formation of wz inp phase has been observed as a polytype in 
 nanowires growing along the   direction . its formation is most 
 likely due to the lower surface energy of the parallel side facets 
 of the wz wires compared to that of the zb ones , and the interface 
 energies involved in the vls three - phase technique . we therefore report here for the first time a colloidal 
 approach to obtain wz inp ncs . theoretical calculations on bulk cu3-xp and experimental evidence from different techniques 
 indicate that cu3-xp ncs are plasmonic 
 and that the lspr arises from the collective motion of positive charge 
 carriers ( holes ) . we have studied , by means of ta measurements , the 
 plasmon dynamics of cu3-xp ncs 
 ( from which a possible use of these ncs for ultrafast nir switching 
 can be envisaged ) . it is possible that the plasmonic 
 behavior stems from the presence of a large number of cu vacancies 
 in the ncs ( on average one cu vacancy per unit cell at room temperature ) . the presence of vacancies , and the partial ionic character of the 
 cu3-xp lattice ( as calculated by 
 us ) , is perhaps the reason for the ability of such ncs to undergo 
 cation exchange to inp . the exchange proceeds through cu3-xp / inp intermediated heterostructures ( given the immiscibility 
 of cu3-xp and inp ) and is likely 
 mediated by cu vacancy diffusion . the close match in the anion frameworks 
 for the two materials entails little distortion of the nc lattice 
 during the transformations and facilitates the formation of single 
 crystalline inp ncs in the wurtzite phase , which is unusual for colloidally 
 synthesized ncs . progress in this direction may come from the exploitation 
 of similar features in cu3as and cu3sb ncs .
synthesis 
 approaches to colloidal cu3p nanocrystals ( ncs ) have been 
 recently developed , and their optical absorption features in the near - infrared 
 ( nir ) have been interpreted as arising from a localized surface plasmon 
 resonance ( lspr ) . 
 our pump  probe measurements on platelet - shaped 
 cu3-xp ncs corroborate the plasmonic 
 character of this absorption . in accordance with studies on crystal 
 structure analysis of cu3p dating back to the 1970s , our 
 density functional calculations indicate that this material is substoichiometric 
 in copper , since the energy of formation of cu vacancies in certain 
 crystallographic sites is negative , that is , they are thermodynamically 
 favored . 
 also , thermoelectric measurements point to a p - type behavior 
 of the majority carriers from films of cu3-xp ncs . 
 it is likely that both the lspr and the p - type character 
 of our cu3-xp ncs arise from the 
 presence of a large number of cu vacancies in such ncs . motivated 
 by the presence of cu vacancies that facilitate the ion diffusion , 
 we have additionally exploited cu3-xp ncs as a starting material on which to probe cation exchange reactions . 
 we demonstrate here that cu3-xp 
 ncs can be easily cation - exchanged to hexagonal wurtzite inp ncs , 
 with preservation of the anion framework ( the anion framework in cu3-xp is very close to that of wurtzite 
 inp ) . 
 intermediate steps in this reaction are represented by cu3-xp / inp heterostructures , as a consequence 
 of the fact that the exchange between cu+ and in3 + ions starts from the peripheral corners of each nc and gradually 
 evolves toward the center . 
 the feasibility of this transformation 
 makes cu3-xp ncs an interesting 
 material platform from which to access other metal phosphides by cation 
 exchange .