Reversible Energy Transfer Research Articles

Triplet Energy Gap-Regulated Room Temperature Phosphorescence in Host-Guest Doped Systems.

The organic room temperature phosphorescence (RTP) materials via host-guest doped method receive considerable attention in the fields of optoelectronics, bioimaging, and information encryption. Despite many host-guest doped materials with excellent RTP properties have been developed, their luminous mechanism is still limited. Here, a series of host-guest doped materials, using benzophenone as the host and quinone compounds as the guests, were constructed to investigate the effect of the triplet energy gap (ΔET) between the host and guest on triplet states population. The guest's triplet state is proposed to be a "triplet energy reservoir", gathering the triplet excitons to emit RTP when ΔET is large and returning triplet excitons to the host when ΔET is small. By combining the results of steady-state and delayed emission spectra, time-resolved transient absorption spectra, and theoretical calculations, a bidirectional energy transfer process is proved, which are triplet-triplet energy transfer and reverse triplet-triplet energy transfer processes. The thermal equilibrium of these two energy transfer processes can be regulated by the ΔET and temperature. The potential applications of these RTP properties are also realized in data encryption and anti-counterfeiting. This work provides valuable insight into the design of host-guest doped materials based on energy transfer mechanisms.

Second-sphere interactions promote antenna effects for selective sensing of DPA in TbIII-based organic frameworks

Investigating the emission patterns of Ln-metal-organic frameworks (MOFs) is crucial but challenging for rational design and selecting suitable MOFs. Firstly, theoretical calculation was used to predict the energy transfer efficiencies between the central ions (Eu3+/Tb3+) and the ligand (H3BTTC). The result indicates that Eu3+ can be completely sensitized and therefore Eu-BTTC with high energy transfer efficiency shows single-emission, whereas Tb3+ can only be partially sensitized and Tb-BTTC shows dual-emission due to reverse energy transfer. Subsequently, Eu-BTTC and Tb-BTTC with heterogeneous emission properties were successfully constructed. In order to investigate the emission modes of the two LnMOFs, 2,6-dipyridyl acid (DPA) was chosen as the second sphere and triggered the interaction between DPA and MOFs. In Eu-BTTC, the fluorescence remained constant before and after the addition of DPA due to the Eu3+ sensitized completely. In Tb-BTTC, the interaction between MOF and DPA improves the energy transfer efficiency from BTTC to Tb3+, resulting in the fully sensitization of Tb3+ and a significant enhancement of the fluorescence of Tb3+. In response to these results, a series of experiments as well as theoretical simulations were conducted to reveal the mechanism. A rational design mode and a novel sensing mechanism for the sensing platform of Ln-MOFs was provided by this study.

Modulation of the wake of a horizontal cylinder by pycnocline thickness in stratified flow

This study investigates the modulating effect of the pycnocline thickness in two-dimensional stratified flow on a cylinder. It encompasses three typical flow regimes identified by Boyer in experiments conducted within the Reynolds number range of Re = 260–2000. Quantitative control of the modulation effect of internal interface waves on the cylinder wake is achieved by varying the thickness of the pycnocline. Under appropriate thickness of the pycnocline, a multiple-centerline-structure can transition to isolated-mixed-structure flow regimes and Double-Eddy-Wavy-Wake flow regimes. Similar modulation patterns are also observed in isolated-mixed flow regimes. Normalized pressure distribution and velocity fields indicate that in low Reynolds number flow regimes (Re < 600), downstream isolated mixed regions generate dynamic pressure that periodically cascades upstream. This periodic reverse energy transfer provides favorable adverse pressure gradients, cyclically reducing the drag force on the cylinder. The cyclic period is, for the first time, classified into four stages: dynamic pressure storage stage, dynamic pressure transfer stage, dynamic pressure consumption stage, and dynamic pressure exhaustion stage. Despite the highly nonlinear modulation effect of internal interface waves in low Reynolds number conditions, the linear predictive theory of lee waves based on streamline equations remains instructive in predicting the trend of lee waves wavelength variation with pycnocline thickness. Drawing upon the modulation study results concerning the pycnocline thickness on lee waves, a regime map is constructed, illustrating the directional evolution of lee waves flow patterns based on variations in pycnocline thickness.

Terpyridyl-Imidazole Based Ligand Coordinated to Ln(Hexafluoroacetyl acetonate)3 Core: Synthesis, Structural Characterization, Luminescence Properties, and Thermosensing Behaviors in Solution and PMMA Film.

A new array of ternary lanthanide complexes of the form [Ln(hfa)3(tpy-HImzphen)] have been synthesized and thoroughly characterized wherein Ln = LaIII (1), EuIII (2), SmIII (3), and TbIII (4); hfa = hexafluoroacetylacetonate; and tpy-HImzphen = 2-(4-[2,2':6',2″]terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole. Incorporation of tpy-HImzphen onto the Ln-hfa moiety induced a bathochromic shift of the absorption window of the complexes into the visible region. Extensive investigations of the luminescence characteristics have been conducted both at RT and at 77 K to understand the deactivation pathways of the complexes. Both steady-state and time-resolved emission spectral behaviors indicate four distinctive behaviors upon incorporation of tpy-HImzphen onto the lanthanide core, viz., a huge red-shift of the ligand-centered peak for LaIII; almost complete energy transfer for EuIII; very little energy transfer for SmIII, while reverse energy transfer in the case of TbIII. In addition, the EuIII-complex exhibits its excellence in luminescence thermometry in the solution state as well as in poly(methyl methacrylate) (PMMA) thin films. The thermosensitive luminescence response in solution was further utilized to mimic set-reset flip-flop logic operation. A plausible energy transfer scheme has been devised to explain dissimilar luminescence behaviors in the complexes. The role of LMCT was also considered for the observed thermosensing property of the Eu(III) complex.

Dy-Al-Ce codoped silica: a promising yellow laser glass with enhanced resistance to X-ray-induced photodarkening

To develop Dy doped silica glass with a higher irradiation resistance, which can be adapted to high power violet or blue LD pumped yellow laser fibers, the designed Dy-Al-Ce codoped silica glasses, 0.05Dy2O3-1.5Al2O3-xCe2O3-(98.45-x)SiO2 (x = 0, 0.05, 0.1, 0.25, 0.5), were prepared by the sol-gel method. Their excitation spectra, emission spectra and emission decay curves associated with the yellow emission from 4F9/2 to 6H13/2 of Dy were determined before and after the X-ray irradiation of 1000 Gy. The relation between these spectra and Ce-codoping concentrations is discussed, including the sensitization from Ce to Dy, the reverse energy transfer from Dy to Ce, and especially the X-ray-induced photodarkening, which is detrimental to the 576 nm yellow emission of Dy. The centers that cause the photodarkening are analyzed by electron paramagnetic resonance and radiation induced absorption spectra. It is found that Ce can effectively suppress the Al-oxygen hole center induced by the X-ray across the entire concentration range of Ce-codoping, but a new photodarkening center is generated at higher concentrations of Ce-codoping. Finally, the optimized Ce-codoping concentration of ∼0.1 mol% is used to achieve a promising yellow laser glass of Dy-Al-Ce codoped silica with enhanced irradiation resistance, resulting in its X-ray-induced photodarkening that is only 6% - 14% of that in the Ce-undoped.

Radical Spin Polarization and Magnetosensitivity from Reversible Energy Transfer.

Molecular spins provide potential building units for future quantum information science and spintronic technologies. In particular, doublet (S = 1/2) and triplet (S = 1) molecular spin states have the potential for excellent optical and spin properties for these applications if useful photon-spin mechanisms at room temperature can be devised. Here we explore the potential of exploiting reversible energy transfer between triplet and doublet states to establish magnetosensitive luminescence and spin polarization. We investigate the dependence of the photon-spin mechanism on the magnitude and sign of the exchange interaction between the doublet and triplet spin components in amorphous and crystalline model systems. The design of a magnetic field inclination sensor is proposed from understanding the required "structure" (spin interactions) to "function" (magnetosensitivity).

Achieving Multiwavelength Optical Amplification Based on Polymer Waveguides Doped with NaYF4:Er3+, Yb3+ Nanoparticles under Commercial and Convenient Led Pumping

AbstractRelying on a reverse energy transfer mechanism between erbium (Er3+) and ytterbium (Yb3+) ions, optical gains at wavelengths of 1550, 980, and 1067 nm are achieved in NaYF4: Er3+, and Yb3+ nanoparticles‐doped polymer waveguides under excitation of low‐power light‐emitting diodes (LEDs) instead of 976 or 1480 nm semiconductor laser as pump sources. The optical gains at 1550, 980, and 1067 nm are demonstrated in five waveguide structures: active‐rectangular, evanescent‐field with and without an aluminium (Al) reflector, and dual‐active hybrid with and without an Al reflector. The maximum relative optical gains of 6.6, 4.0, and 3.8 dBcm−1 are realized at 1550, 1067, and 980 nm, respectively, in a 5 mm long dual‐active hybrid waveguide with cross‐section of 4 × 10 µm, and a ≈100 nm‐thick Al reflector grown under the lower cladding layer under excitation of a 395 nm LED. This work proves the feasibility of using a low‐cost LED to realize multi‐wavelength amplification in a dense wavelength‐division multiplexing system and is thus valuable for the development of complex multi‐functional photonic‐integrated‐module mixed‐band optical communication systems.

Highly Effective Near‐Infrared Light‐Activatable Triplet Fusion Upconversion with BODIPY Annihilators: Triplet Modulation via Non‐Conjugated n–π Spatial Electron Coupling

AbstractEffective strategies to manage the triplet state of chromophores are desirable in terms of their crucial role in photonic and optoelectronic technologies. However, triplet excited‐state energy level regulation of organic molecules remains elusive. Here, non‐conjugated n–π spatial electron coupling is demonstrated within boron‐dipyrromethene (BODIPY) and can greatly reduce the low‐lying triplet state (T1) to 1.34 eV without the conventional need for π‐conjugation extension. Moreover, the close alignment between T1 states of this BODIPY derivative and the photosensitizer PtTNP contributes to the reversible triplet energy transfer, namely the intermolecular triplet excited‐state equilibrium. Furthermore, to the best of the knowledge, the first BODIPY‐based near‐infrared light‐activated triplet fusion upconversion (NIR TF‐UC) with an upconversion quantum efficiency of 13.3% is implemented. On this basis, the air‐stable solid‐state NIR TF‐UC material is successfully fabricated, whose upconverted emission can still be seen at an ultralow NIR power density of 6.4 mW cm−2. This new strategy to implement triplet state modulation will accelerate the development of NIR TF‐UC and other important areas involving triplet state, such as photoredox catalysis, photodynamic therapy, and photovoltaic devices.

Dynamic Assembly‐Induced Time‐Resolved Optical Switches for Rewritable Advanced Information Encryption

AbstractThe development of photocontrolled assembly of smart materials with time‐dependent optical characteristics is a promising strategy to fabricate multiple optical materials for advanced information encryption. Herein, this work reports the integration of photochoromic spiropyran and square‐planar platinum(II) as a photocontrolled switching motif that endows the materials with dynamic assembly‐induced optical characteristics. Importantly, the time‐resolved multilevel optical switching processes have been achieved based on the synergism of the reversible photochoromism and fluorescence resonance energy transfer (FRET) processes through regulating the intramolecular isomerization and intermolecular self‐assembly processes. The excellent temporal multicolor features promote the photocontrolled switches as a promising candidate for time‐resolved confidential materials including multilevel displays and dynamic 4 dimension (4D) codes, with the achievement of relative encrypted data that can only be identified at a specified time. This synergistical strategy for constructing advanced materials provides an inspiring guidance for information encryption with higher security requirements.

Cooperative Energy Transfer between Two Neighboring Eu3+ Ions and Energy Transfers from Eu2+ to Eu3+ and from Eu3+ to Yb3+ in Crystals for Quantum Cutting

Energy transfer between Eu2+ and Eu3+ ions is examined in CaF2 crystal at not only room temperature but also 12 K. Energy transfer from Eu2+ to Eu3+ is confirmed. The reverse energy transfer from Eu3+ to Eu2+, however, is confirmed to be undetectable. Cooperative energy transfer between two neighboring Eu3+ ions in Eu3+-doped LiYb1−xEux(MoO4)2 crystal, which is combined with Eu-O charge transfer complex resonantly, is found to be responsible for the 1000 nm near infrared broadband emission from Yb3+ ions through the quantum cutting from the 5D2 excited state of Eu3+.

Mn Doping in Low-Dimensional Perovskites: A Switch for Controlling Dopant and Host Emission

The growth of solid-state photonics relies on the design of highly efficient devices with tunable photoluminescence (PL) based on thermally stable phosphors. In this study, a room-temperature, all-ambient synthesis of Mn-doped hybrid lead halide perovskite phosphors is presented. By applying temperature-dependent (4.2–300 K) steady-state and transient PL measurements, the photophysical pathways of the hybrid system were identified and the carrier recombination processes at play were defined. At higher Mn-doping concentrations (>4%), the excitonic white light host emission nearly disappears, and a bright Mn transition appears at ∼650 nm with a record PL quantum yield of ∼87.4%. The intense sensitized Mn luminescence results from rapid and efficient exciton-to-dopant energy transfer, as evidenced by time-gated and femtosecond fluorescence upconversion measurements. A millisecond PL decay time attributed to the Mn dopant and a microsecond PL decay time associated with the excitonic emission of the host were found, indicating energy and reverse energy transfer in the system. These findings provide unique insights into the chemical tailoring of the Mn-dopant emission by investigating the coupling between the host semiconductor and the dopant ion, a stepping stone toward the development of high-performance solid-state light-emitting diodes and scintillators.

Photoluminescence of Tb3+/Sm3+ ions co-activated in SrO - Na2O - WO3 - tellurite glasses for multicolor laser applications

A novel series of Tb3+, Sm3+ single doped and Tb3+/Sm3+ co-incorporated tungsten tellurite glasses were synthesized by melt quenching technique and corresponding structural as well as luminescence features of the prepared glasses have been reported here. Spectral overlapping between the luminescence spectra of Tb3+ ions and the excitation spectra of Sm3+ ions manifests that the energy transfer process takes place from Tb3+ ions to Sm3+ ions. By using the dual excitations at 377 and 484 nm, the titled co-doped glasses emit green light of wavelength 542 nm along with reddish – orange colour light at 599 nm. In addition to this, there is no possibility of reverse energy transfer which is validated with the help of excitation at 403 nm (Sm3+ ions) as major evidence. The lifetimes of all co-doped glasses decline with increasing Tb3+ doping level in the ligand matrices, indicating the energy migration process takes place from Tb3+→ Sm3+. The chromaticity coordinates of all synthesized co-doped glasses lie in yellowish-orange region of CIE1931 diagram and it shifts to deep yellow region when Tb3+ ion concentration varies. Our findings propose that the titled glasses can be used as visible laser materials for multicolor laser applications.

Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

Emissive compounds with long emission lifetimes (μs to ms) in the visible region are of interest for a range of applications, from oxygen sensing to cellular imaging. The emission behavior of Ir(ppy)2(acac) complexes (where ppy is the 2-phenylpyridyl chelate and acac is the acetylacetonate chelate) with an oligo(para-phenyleneethynylene) (OPE3) motif containing three para-rings and two ethynyl bridges attached to acac or ppy is examined here due to the accessibility of the long-lived OPE3 triplet states. Nine Ir(ppy)2(acac) complexes with OPE3 units are synthesized where the OPE3 motif is at the acac moiety (aOPE3), incorporated in the ppy chelate (pOPE3) or attached to ppy via a durylene link (dOPE3). The aOPE3 and dOPE3 complexes contain OPE3 units that are decoupled from the Ir(ppy)2(acac) core by adopting perpendicular ring-ring orientations, whereas the pOPE3 complexes have OPE3 integrated into the ppy ligand to maximize electronic coupling with the Ir(ppy)2(acac) core. While the conjugated pOPE3 complexes show emission lifetimes of 0.69-32.8 μs similar to the lifetimes of 1.00-23.1 μs for the non-OPE3 Ir(ppy)2(acac) complexes synthesized here, the decoupled aOPE3 and dOPE3 complexes reveal long emission lifetimes of 50-625 μs. The long lifetimes found in aOPE3 and dOPE3 complexes are due to intramolecular reversible electronic energy transfer (REET) where the long-lived triplet-state metal to ligand charge transfer (3MLCT) states exchange via REET with the even longer-lived triplet-state localized OPE3 states. The proposed REET process is supported by changes observed in excitation wavelength-dependent and time-dependent emission spectra from aOPE3 and dOPE3 complexes, whereas emission spectra from pOPE3 complexes remain independent of the excitation wavelength and time due to the well-established 3MLCT states of many Ir(ppy)2(acac) complexes. The long lifetimes, visible emission maxima (524-526 nm), and photoluminescent quantum yields of 0.44-0.60 for the dOPE3 complexes indicate the possibility of utilizing such compounds in oxygen-sensing and cellular imaging applications.

Thermally Activated Delayed Near‐Infrared Photoluminescence from Functionalized Lead‐Free Nanocrystals

AbstractAs an analogue to thermally activated delayed fluorescence (TADF) of organic molecules, thermally activated delayed photoluminescence (TADPL) observed in molecule‐functionalized semiconductor nanocrystals represents an exotic mechanism to harvest energy from dark molecular triplets and to obtain controllable, long‐lived PL from nanocrystals. The reported TADPL systems have successfully covered the visible spectrum. However, TADF molecules already emit very efficiently in the visible, diminishing the technological impact of the less‐efficient nanocrystal‐molecule TADPL. Here we report bright, near‐infrared TADPL in lead‐free CuInSe2 nanocrystals functionalized with carboxylated tetracene ligands, which results from efficient triplet energy transfer from photoexcited nanocrystals to ligands, followed with thermally activated reverse energy transfer from ligand triplets back to nanocrystals. This strategy prolonged the nanocrystal exciton lifetime from 100 ns to 60 μs at room temperature.

Thermally Activated Delayed Near-Infrared Photoluminescence from Functionalized Lead-Free Nanocrystals.

As an analogue to thermally activated delayed fluorescence (TADF) of organic molecules, thermally activated delayed photoluminescence (TADPL) observed in molecule-functionalized semiconductor nanocrystals represents an exotic mechanism to harvest energy from dark molecular triplets and to obtain controllable, long-lived PL from nanocrystals. The reported TADPL systems have successfully covered the visible spectrum. However, TADF molecules already emit very efficiently in the visible, diminishing the technological impact of the less-efficient nanocrystal-molecule TADPL. Here we report bright, near-infrared TADPL in lead-free CuInSe2 nanocrystals functionalized with carboxylated tetracene ligands, which results from efficient triplet energy transfer from photoexcited nanocrystals to ligands, followed with thermally activated reverse energy transfer from ligand triplets back to nanocrystals. This strategy prolonged the nanocrystal exciton lifetime from 100 ns to 60 μs at room temperature.

Cool white light and tunable multicolor emission from Tb3+/Dy3+ co-activated glasses under different excitations for WLEDs

A set of co-doped (Tb3+/Dy3+) lithium zinc borate (LZB) glasses were developed by melt quenching. The structural evaluation was performed for synthesized glassy matrices. The Dy3+ and Tb3+ individually doped glasses exhibit intense yellow and green luminescence bands at 575 nm (4F9/2 → 6H13/2) and 543 nm (5D4→7F5), respectively. The sensitization effect of Dy3+ on Tb3+ was analyzed by increasing the Tb3+ content with respect to the optimum Dy3+ content (0.5 mol%) in Dy3+/Tb3+. The spectral overlay of Dy3+ luminescence and Tb3+ absorption profiles, Dy3+/Tb3+ PL spectra under different excitations 352, 362, 376, and 385 nm, shortening decay lifetimes of Dy3+ in Dy3+/Tb3+ co-activated glasses, energy transfer (ET) parameters, chromaticity coordinates and their corresponding correlated temperatures all help to explain ET from Dy3+ to Tb3+. At 385 nm of Dy3+ excitation, the optimized co-activated (0.5Dy3++1.0Tb3+):LZB glass displays cool white light emission. The non-radiative ET from Dy3+ to Tb3+ is dominated by electric dipole–dipole interaction and its ET efficiency was calculated to be 63%. At the same time, reverse ET from Tb3+ to Dy3+ was also analyzed. The shift in color coordinates from dominant yellow to greenish-yellow, green and white light emission suggests that Dy3+/Tb3+ co-activated LZB glasses can be a potential candidate for UV converted multicolor and white light emitting devices.

Near-infrared rechargeable glass-based composites for green persistent luminescence

The fabrication of Yb3+, Tm3+ co-doped oxyfluorophosphate glass-based composites, with green persistent luminescence after being charged with near-infrared light, is demonstrated. The mechanism responsible for the green afterglow after near-infrared illumination is unveiled. The composite is prepared using a modified melting process to limit the evaporation of fluorine during melting. Intense (blue and ultraviolet) up-conversion emission is obtained by optimizing the Yb2O3 and Tm2O3 concentrations. A heat treatment promotes volume precipitation of Yb3+, Tm3+ co-doped CaF2 crystals. Although the intensity of the blue up-conversion emission from the Tm3+1G4 level is lower in the highly Yb3+-concentrated glass-ceramic due to reverse energy transfer from Tm3+ to Yb3+, the heat treatment leads to an increase of the intensity of the emissions around 346 nm, 361 nm nm and 450 nm coming from the Tm3+1I6 and 1D2 levels. By combining the Yb3+ and Tm3+ ions with SrAl2O4:Eu2+,Dy3+crystals, green afterglow can be obtained after charging with near-infrared light.

Energy Transfer Processes in the Excited States of an {[Ir(N C)2(N N)]+‐Rhodamine} Dyad: An Experimental and Theoretical Study

AbstractHerein we report the synthesis, characterization, photophysical and theoretical study of the luminescent dyad Ir‐RhB containing two emissive centers: The first fluorescent (RhB=rhodamine B) and the second phosphorescent (Ir=orthometalated iridium complex), bound with an aliphatic bridge. These nearly independent chromophores have triplet excited states which are very close in terms of energy, allowing for reversible energy transfer between them. The observed transient absorption spectra and their time dependent variations were interpreted in terms of a relatively simple kinetic model, where selective excitation of the iridium‐based chromophore affords the corresponding emissive triplet state, followed by its equilibration with the rhodamine based triplet state and simultaneous emissive relaxation to the dyad ground state. This model allows for analytical solution to give key characteristics of the dyad excited‐state dynamics, including the energy gap between these triplets (ΔE=0.017 eV) and the rate constant of energy transfer (k(Ir‐RhB)=6.0 ⋅ 108 s−1). These data were also verified by quantum chemical DFT and TD‐DFT calculations.

Abnormal Hot Carrier Decay via Spin-Phonon Coupling in Intercalated van der Waals Ferromagnetic Fe1/3TaS2.

Spin-phonon coupling is a fundamental interaction in ferromagnets/antiferromagnets and plays a key role in hot carrier decay. Normally, spin transfers its excess energy to a lattice via spin-phonon coupling in hot carrier decay in ferromagnets/antiferromagnets. However, the reverse energy transfer process (i.e., from lattice to spin) is feasible in principle but rarely reported. Here, we observe an abnormal hot carrier decay with a slow fall (80 ps) in ΔR(t)/R0 time series in ferromagnet Fe1/3TaS2, which is a result of the lattice of TaS2 vdW layer transfering its energy to spin via spin-phonon coupling. The Fe ions inserted between TaS2 vdW layers with very weak bonding with TaS2 vdW layer, are the origin of the ferromagnetism and give rise to its weak electron-spin and spin-phonon couplings which in turn lead to the observed abnormal hot carrier decay in the ferromagnetic phase Fe1/3TaS2.

Effect of wall temperature on the kinetic energy transfer in a hypersonic turbulent boundary layer

The effect of wall temperature on the transfer of kinetic energy in a hypersonic turbulent boundary layer for different Mach numbers and wall temperature ratios is studied by direct numerical simulation. A cold wall temperature can enhance the compressibility effect in the near-wall region through increasing the temperature gradient and wall heat flux. It is shown that the cold wall temperature enhances the local reverse transfer of kinetic energy from small scales to large scales, and suppresses the local direct transfer of kinetic energy from large scales to small scales. The average filtered spatial convection and average filtered viscous dissipation are dominant in the near-wall region, while the average subgrid-scale flux of kinetic energy achieves its peak value in the buffer layer. It is found that the wall can suppress the inter-scale transfer of kinetic energy, especially for the situation of a cold wall. A strong local reverse transfer of fluctuating kinetic energy is identified in the buffer layer in the inertial range. Helmholtz decomposition is applied to analyse the compressibility effect on the subgrid-scale flux of kinetic energy. A strong transfer of the solenoidal component of fluctuating kinetic energy is identified in the buffer layer, while a significant transfer of the dilatational component of fluctuating kinetic energy is observed in the near-wall region. It is also shown that compression motions have a major contribution to the direct transfer of fluctuating kinetic energy, while expansion motions play a marked role in the reverse transfer of fluctuating kinetic energy.